This paper first theoretically describes the micro mechanisms through which FDI inflow affects the export diversification of domestic enterprises, and then conducts an empirical study using Chinese firm-level micro data. Results show that FDI inflow promotes enterprises’export diversification significantly. Mechanism tests indicate that FDI inflow improves export diversification through channels including financing constraints alleviation, R&D innovation promotion and OFDI promotion. Heterogenous analysis show that the impact of FDI inflow on export diversification is more obvious among private enterprises, ordinary trade and enterprises with higher absorption capacity. Finally, this paper further distinguishes the types and destinations of export diversification, and finds that the promotion effect of FDI inflow on export diversification is mainly manifested in export market diversification, and plays a more prominent role in the process of exporting to developing countries. This paper examines the relationship between FDI inflow and export diversification comprehensively, which not only provides a strong micro basis for the objective evaluation of the economic effects of China’s investment policy, but also provides beneficial policy enlightenment for China to reduce foreign trade risks and promote the high-quality development of trade.