摘要：Unclear rules lead to uncertainty, moderating the effectiveness of public policies and causing large deadweight losses. China piloted a reform of its permit policy in 2016, aiming to integrate existing environmental regulations to make the environmental regulation more clear. We use the reform of the pollutant discharge permit system in China as a quasi-natural experiment to evaluate the impact of clear rules on policy performance. Using unique firm-level data and the difference-in-differences-in-differences (DDD) approach, we find that a clear policy leads firms to reduce the emission concentrations of major pollutants, chemical oxygen demand (COD), ammonia nitrogen (NH3-N), sulfur dioxide (SO2), and nitrogen oxides (NOX), by 7.70%, 28.97%, 19.51%, and 12.01%, respectively. Compared with large and state-owned enterprises, small and non-state-owned enterprises reduced more emissions after permits were issued. The results suggest that clear rules can facilitate compliance and that governments should design clearer environmental regulations and policies in future environmental governance.