Against the backdrop of rapid global sustainable development and green finance, the quality of Environment, Society, and Governance (ESG) information disclosure has increasingly garnered attention from regulators and investors. However, the phenomenon of companies engaging in ESG greenwashing through symbolic disclosure is becoming increasingly common, which impairs capital market resource allocation efficiency and hampers the clear transmission of policy signals. This study takes the implementation of the 2018 Environmental Protection Tax Law as an exogenous shock and utilizes data from Chinese A-share listed companies to empirically test how the introduction of environmental protection tax affects corporate ESG greenwashing behavior using the Difference in Differences (DID) method. The findings indicate that the enforcement of environmental protection tax notably curtails such greenwashing behavior, and this outcome has been consistently confirmed through various robustness checks. Moreover, further mechanism analysis reveals that environmental protection tax indirectly curbs greenwashing behavior by increasing analyst attention, easing corporate financing constraints, and encouraging companies to reduce symbolic disclosure. Further, results from the heterogeneity analysis demonstrate a significantly stronger effect of environmental protection tax governance in the eastern region, high tax burden areas, and enterprises with high ESG scores. This paper explores the economic impacts of environmental protection tax from the perspectives of signal transmission and compliance, enriching the research path of external governance mechanisms for ESG greenwashing behavior. At the same time, using a text similarity index based on natural language processing to quantify the degree of greenwashing provides a new measurement tool for ESG research.