Protecting the legitimate rights and interests of investors, especially small and medium-sized investors, is an inevitable requirement for the high-quality development of the capital market. Taking A-share non-financial listed companies in Shanghai and Shenzhen from 2011 to 2022 as the research object, we empirically examine the impact of ESG ratings on major shareholders' tunneling behaviors and its influence mechanism from the perspectives of capital appropriation and connected transactions. It is found that ESG ratings can significantly inhibit major shareholders' tunneling behaviors. The impact mechanism test shows that ESG ratings can inhibit major shareholders' tunneling behaviors by enhancing information transparency (incentivizing internal disclosure and attracting analysts'attention), strengthening the threat of exit for small and medium-sized shareholders, and improving the quality of internal control in order to inhibit capital misappropriation type of tunneling behaviors. Heterogeneity analysis finds that ESG ratings have a more significant inhibitory effect on major shareholders'tunneling behaviors in a high-quality legal environment and high equity checks and balances, and that ESG ratings can more significantly inhibit major shareholders' tunneling behaviors of the company through connected transactions in state-owned enterprises. The findings provide empirical evidence for strengthening shareholder governance and investor protection, and have great significance for corporate ESG practices and the high-quality development of the capital market.