This study empirically investigated the effect of adjustment of the China Securities Index 300 (CSI 300) on environmental information disclosure (EID) by index constituents, based on propensity score matching and difference-in-difference approaches. The results showed that the inclusion in the CSI 300 significantly improved the quality of EID by firms. Moreover, this positive impact was more pronounced among firms with lower agency costs and those located in regions with a stronger legal environment. Further testing of the mediating mechanism revealed that becoming an index constituent served to curb opportunistic behavior by managers arising from shortsightedness. Our results were valid after addressing the potential endogeneity between index adjustment and EID and remained unchanged in various other robustness tests. The findings provide support for the positive impact of stock market index adjustment on non-financial information disclosure and have practical implications for decision-making regarding EID in China and other emerging markets. •CSI 300 inclusion improved the quality of corporate EID.•The above effect was moderated by agency costs and legal environment.•Reduction of managerial opportunistic behavior is the mediating mechanism.•Findings support index adjustment’ impact on non-financial information disclosure.•Findings have practical implications for decision-making regarding EID.