How to improve the sustainable development performance of enterprises has become a hot topic in both academic and practical fields. In the VUCA era, financial flexibility has given new connotations and significance to this topic. This paper takes China’s Ashare listed companies from 2009 to 2022 as research samples to explore the nonlinear impact of financial flexibility on the sustainable development performance of enterprises from theoretical and empirical levels, and deeply analyzes its internal mechanism and boundary conditions. The empirical evidence suggests an inverted Ushaped relationship between financial flexibility and sustainable development performance. With the transformation of financial flexibility from low to high, sustainable development performance presents a nonlinear evolution trend of first rising and then falling. That is, financial flexibility plays a “stepping stone role” in promoting sustainable development performance when appropriate and a “stumbling block effect” when excessive. Mechanism testing shows that financial flexibility mainly affects sustainable development performance through the three channels of organizational resilience, persistent green innovation and investment efficiency. Moderating effect test shows that unabsorbed slack weakens the inverse Ushaped relationship between financial flexibility and sustainable development performance, while environmental uncertainty strengthens the inverse Ushaped relationship between them. Finally, further heterogeneity analysis indicates that financial flexibility has a greater enhancing effect and a larger scope of influence on the sustainable development performance of heavily polluting industries and nonstateowned enterprises.