Waste threatens human health and the environment. Public participation, as an essential link in the whole waste management chain, is important for countries to achieve carbon neutrality and promote sustainable development. However, some countries are still in the initial stage of policy practice, for example, China’s waste sorting policy is still in the pilot stage and has not yet covered the whole country. Therefore, it is necessary to understand whether waste sorting policy has a catalytic effect on public waste sorting behavior, and based on the Theory of Planned Behavior, knowledge and intention were proposed to have chain-mediating effect. With a large-sample national survey, we compared the differences in public waste sorting behavior between areas with and without waste sorting policies and examined the mechanism. Results showed that the public in areas with waste sorting policies participated more in actual waste sorting behavior than the public in areas without waste sorting policies, and the proposed effects of knowledge and intention were verified. The research results provide valuable insights for policymakers and stakeholders from multiple perspectives of policy implementation, policy instruments, and population differences.