With economic globalization and the Chinese economy entering new normal，the trend of“China going west”is becoming obvious． International diversification ( DIV) has become one of the strategic choices for many Chinese export enterprises． How does DIV affect the export performance? Based on the institutional theory，a theoretical model of how international diversification affects firms’export performance is constructed． Private listed companies are used as data to test the hypotheses． It is found that DIV inhibits rather than promotes the export performance，which is exacerbated by heterogeneous resources and mitigated by a well-established institutional environment． Thus，an improved institutional environment is a necessary condition for DIV to promote export performance． If the institutional environment is poor，the heterogeneity of resources is stronger; DIV results in a greater negative impact on the export performance．