Taking China’s environmental “fee-to-tax” reform as a quasi-natural experiment, this paper employs a difference-in-differences (DID) approach to examine the impact of the green tax reform on firms’ cost of equity on a sample of A-share listed firms from 2012 to 2023. The results show that the implementation of the Environmental Protection Tax Law significantly reduced the cost of equity for heavily polluting firms. This finding remains robust after a series of robustness checks. Analyses of the underlying mechanisms reveal that the green tax reform lowered the cost of equity primarily through the channel of “Porter effect.” Moreover, the effect is more pronounced among firms located in the eastern China, those with stronger internal governance, and those operating in a stricter external monitoring environment.