Through the selection of cross-border data in 2008， 2010， 2012 and 2015， this paper studies the mechanism of restraining budget opportunism by financial audit reports. The research finds that the quality characteristics of financial audit report， such as timeliness， comprehensiveness， clarity and rectification response， can play an information appraisal role to directly restrain budget opportunism. Feedback information from financial audit results to legislative organs and the public can play a signaling role to indirectly restrain budget opportunism by improving legislative supervision ability and public participation. In the countries with strong corruption control ability, the mechanism of information appraisal and signal transmission play a role at the same time. In the countries with weak corruption control ability, the mechanism of information appraisal is more significant, but the mechanism of signal transmission is limited. The research conclusion is of great significance for our auditing institutions to adhere to the leadership of the Central Audit Committee， improve the quality of financial audit reports， report to the National People's Congress and publish to the society for continuously restraining budget opportunism under the current high-pressure situation of anti-corruption.