It is well-acknowledged that digital transformation is significant for both enterprises and society, but usually the enterprise's own resources and capabilities are insufficient, and digital transformation has the characteristics of a high transformation cost, a long transformation period, and a high transformation failure risk. Therefore, in the face of the new development stage, how to use external resources and forces to solve the problems in digital transformation has become critical to promoting high-quality economic development and implementing the "digital China" strategy.#br#Existing studies have explored the driving factors of enterprise digital transformation from the perspectives of government policy support, business environment improvement, enterprises' own resources and capabilities, and the heterogeneity of enterprise management, but ignored the role of strategic alliance as an external force. Strategic alliance is a key channel to acquire external knowledge, and it greatly expands the boundaries of enterprises and disposable resources, helps enterprises obtain heterogeneous information, knowledge and resources related to digital transformation from external sources, and can also reduce the cost and risk of individual enterprises' digital transformation. This will alleviate the problem of insufficient resources and capabilities for digital transformation. However, the existing literature has not yet focused on the role of strategic alliances in driving the digital transformation of enterprises.#br#Thus, taking Shanghai-Shenzhen A-share listed enterprises from 2009 to 2021 as the research object, this study sorts out the data of strategic alliance announcements of listed enterprises and adopts a large-sample empirical research method to explore the impact and mechanism of enterprises' participation in strategic alliance on digital transformation. The heterogeneity of strategic alliance's impact on enterprise digital transformation is further examined from the perspectives of enterprise nature, industry attributes, and additional environmental factors. There are three main findings. First, the participation of enterprises in strategic alliances is conducive to promoting the process of digital transformation and equity-based strategic alliances; the alliance objects are enterprises and governments, and the cooperation content of the alliance involves digital technology, which will strengthen the promotion role of strategic alliances on the digital transformation of enterprises, while the alliance objects are financial institutions and universities,and they have not yet been found to have significant strengthening effects. Second, the participation of enterprises in strategic alliance can improve the level of enterprise risk taking, enhance the innovation ability of enterprises, and reduce the short-sighted behavior of management, thus driving the digital transformation of enterprises. Third, further analysis shows that strategic alliances can effectively play a role in promoting digital transformation for both state-owned and non-state-owned enterprises, high-tech enterprises, and traditional enterprises. It is worth noting that more than any other enterprise, strategic alliances help to promote the digital transformation process of non-state-owned enterprises, high-tech enterprises and enterprises located in regions with a strong culture of business cooperation.#br#The main contributions of this paper lie in three areas. First, different from the existing literature, this paper extends the external inter-organizational relationship factors that affect enterprises' digital transformation to strategic alliance, and discusses the external driving forces of enterprises' digital transformation from the perspective of strategic alliance, providing a new perspective for solving the obstacles and difficulties of enterprises' digital transformation. Second, studies on strategic alliance focus on the theoretical level or case studies. Some scholars adopt empirical research methods to investigate its impact on innovation behavior, production efficiency and enterprise performance, etc., but there is little literature to explore the impact of strategic alliance on enterprise digital transformation. This paper examines the micro-economic consequences of strategic alliances from the perspective of digital transformation, and provides incremental contributions to the research on strategic alliances. Third, under the background of accelerating the development of digital economy and deepening the reform of market-oriented allocation mechanism of factors, linking strategic alliance as a resource allocation optimization mechanism with digital transformation, revealing and testing the driving mechanism and heterogeneous impact of strategic alliance in promoting enterprise digital transformation will not only help deepen the understanding of the role of strategic alliance resource allocation. It can also provide useful reference and inspiration for enterprises how to use external forces to promote digital transformation.#br#