创新既是企业获取长期竞争优势的关键行动，也是我国经济高质量发展的内生动力。董事会作为战略决策机构与治理核心，其内部运作机制必然会影响企业创新水平。基于关系契约理论，以2008—2019年沪深A股上市公司为数据样本，实证研究董事会非正式层级对企业创新的影响。结果发现，董事会非正式层级越清晰，企业创新水平越高，而当股权集中度较高、CEO处于非正式层级最高位时，董事会非正式层级与企业创新间的正相关关系会被削弱。进一步研究发现，董事会非正式层级可以通过降低企业环境不确定性和抑制管理层短视行为提升企业创新水平；区分创新类型后，相比于策略性创新，董事会非正式层级更能够显著提升企业实质性创新水平；董事会非正式层级能够提升企业价值。结论基于新的研究视角从微观动态交互层面理解董事会结构特征对企业创新的影响，对企业加强董事会建设、提升创新水平具有重要借鉴意义。

Innovation is not only the decisive factor for enterprises to obtain a long-term competitive advantage, but also the endogenous driving force for high-quality economic development. The board of directors, as the core of corporate governance, its internal operating mechanism will inevitably affect the enterprise innovation. The sociological theory believes that any group's decision-making will be affected by the hierarchy, either a formal hierarchy or an informal hierarchy. The board of directors follows the voting decision-making rules, and the content of the decision is summary and vague, so it is difficult to be effectively guided by the formal coordination mechanism. When there are internal conflicts and differences, it is more difficult to rely on formal hierarchy to coordinate. In this setting, the informal hierarchy will play a more prominent role in the board of directors' decision-making.#br#The board informal hierarchy is an implicit and informal hierarchical order, which is formed according to the social capital and prestige status owned by director members. The richer the social capital owned by the director member, the stronger the ability to provide information and resources for the enterprise,and this, the higher the degree of respect from other directors, which the director member will be at a higher hierarchy. According to the relationship contract theory, informal institutional relationships established based on trust and respect are more likely to produce supervision and coordination. The informal hierarchy provides the board of directors with an implicit mechanism for orderly communication, enhances the board's resource integration capabilities and communication effects, and enhances the board's decision-making efficiency and supervisory effectiveness. Therefore, theoretically, the board informal hierarchy can improve the level of corporate innovation. #br#This paper selects the data of A-share listed companies from 2008 to 2019 as the research sample. After processing, 20799 observations were obtained. Firstly, it constructs a multiple linear regression model to verify the impact of the board informal hierarchy on enterprise innovation, and investigate the regulatory effect of equity concentration and the CEO position in the board informal hierarchy. Secondly,it builds a mediating effect model to verify the mechanisms of the board informal hierarchy affecting enterprise innovation. Finally,it constructs a probit model to verify the impact of the board informal hierarchy on the types of corporate innovation. In addition, we also verify whether the board informal hierarchy improves the enterprise value.#br#The results show that the clearer the board informal hierarchy, the higher the level of corporate innovation, and when the equity concentration is higher and the CEO is at the highest position in the informal hierarchy, the positive correlation between the board informal hierarchy and corporate innovation is weakened. When the ownership concentration is high, the diversified opinions within the board of directors are reduced, and the informal hierarchy, as an integration mechanism of different views, plays a limited role in innovation decision-making. When the CEO is at the highest hierarchy, the supervision effect of the board of directors on the managers is weakened, the agency conflict is more serious, and the promotion effect of the informal hierarchy on enterprise innovation will be reduced. Further research finds that the board informal hierarchy improves the level of corporate innovation by reducing the environmental uncertainty and the agency conflict of management myopia. After distinguishing the types of innovation, we find that compared with strategic innovation, the board informal hierarchy significantly increases the probability of substantive innovation, and the board informal hierarchy has significantly improved the value of the company. #br#The main contributions of this paper are as follows. Firstly,from the perspective of relational contract theory, this paper promotes the research on board structure and enterprise innovation. Most of the studies regard the internal director members as homogeneous individuals, ignoring the status differences brought by the director members' social capital. Based on the reality of the status differences among directors, this paper studies the impact of board informal hierarchy on enterprise innovation, and enriches the literature of the economic consequences of board informal hierarchy and the influencing factors of enterprise innovation. Secondly, breaking through the static research perspective of the board of directors and based on the dynamic interaction perspective, this paper reveals the mechanisms of board informal hierarchy to improve the enterprise innovation level by reducing the environmental uncertainty and inhibiting the management myopia. At the same time, it is proposed and verified that the board informal hierarchy can improve the substantive innovation level of enterprises. This paper provides a new research perspective for understanding the impact of the structural characteristics of the board of directors on corporate innovation and has important reference significance for companies to optimize the structure of the board of directors and improve the innovation capability.#br#