Abstract: Platform entrying a complementary market in the platform ecosystem has been increasingly concerned by the theoretical and practical community. In order to explore the mechanism of how a platform's entry into a complementary market influences its competitive advantage, this paper studies the cases of JD Logistics and Cainiao Logistics, based on the platform ecosystem theory and grounded theory. The results show that: (1) platform entering a complementary market in an exclusive way will reduce the openness of the platform, resulting in a strong competitive effect between the platform and its complementors. Complementors' service innovation for the platform is reduced, but exclusive entry will enhance the dominant position of the platform, which is beneficial to platform governance; (2) platform entering a complementary market in the open way will enhance the openness of the platform and its complementors' enthusiasm for service innovation, and accelerates the development of the platform and platform ecosystem on the strength of network synergistic effect. The results enrich the platform ecosystem theory, reveal the role of platform openness changes in the process of platform entering complementary markets and provide a theoretical basis for platform's penetration into market and platform ecosystem's governance mechanisms.

Key words: platform, complementary market, platform openness, platform ecosystem