As the leader, CEO plays an important role in corporate strategic decision-making. However, the shortening of the expected tenure will catalyze CEO’s selfish motivation and shortsighted tendency, inducing more diversified decisions, and ultimately damage the long-term interests of enterprises. Based on the Agency Theory and High-echelon Theory, this paper empirically studies the impact of CEO expected tenure on corporate diversification by taking A-share listed non-financial enterprises in China from 2013 to 2020 as samples. The results show that the shortening of CEO expected tenurewill bring more diversification, especially non-related diversification, which is affected by CEO decision-making power and internal and external supervision mechanism. CEO shareholding will strengthen this tendency, and institutional investors’ shareholding and participation in corporate governance can inhibit CEO decision-making power.Compared with non-state-owned enterprises, strict internal decision-making management system of state-owned enterprises can effectively alleviate the impact of short expected tenure on corporate diversification. The external supervision mechanism including analysts and auditors can effectively restrict the diversification tendency of short-term CEOs.