Abstract: As an important part of the national environmental governance and supervision system, government environmental audit is of great significance to promote the green development of the Yangtze River Economic Belt. Taking 110 prefecture-level cities in the Yangtze River Economic Belt as the research scale, the green effect and mechanism of government environmental audit were tested from multiple perspectives. The research showed: (1) Government environmental audit has a significant direct effect on the green development of the Yangtze River Economic Belt, and the breadth of government environment audit is slightly greater than the intensity of government environmental audit; (2) The green effect of government environmental audit in upstream and downstream basins is better than that in midstream basins, and the green governance efficiency of government environmental audit in non-key environmental protection cities is higher than that of key environmental protection cities. In the strengthening phase of the government’s environmental audit work after 2009, its service performance was better than the previous phase; (3) Local government competition, green technology innovation, and industrial structure upgrading are the three major mechanisms for government environmental auditing to indirectly promote the green development of the Yangtze River Economic Belt. The indirect promotion of the intensity of government environmental audit is higher than the breadth of government environmental audit, and the green governance effect exerted by the green technological innovation mechanism is even higher.