Common institutional investors have collusion and governance view, and their impact on audit fees is uncertain. This paper takes Chinese A-share listed companies in Shanghai and Shenzhen from 2007 to 2020 as a sample to empirically examine the impact of joint institutional investors on corporate audit fees. The results show that there is a significant positive correlation between common institutional investors and audit fees, the audit fees with common institutional investors ar higher on average by 3.30％ standard difference. The mechanism test shows that to maximize the value of the portfolio, the common institutional investors will form collusion with the enterprises in the portfolio, increase the real earnings management behavior of enterprises, and improve the information barriers of enterprises in the portfolio. Further analysis shows that when the number of analysts tracking is high, the increase of audit fees caused by common institutional investors is more pronounced; however, the low level of industry competition, senior executives' shareholding and the low pressure of short-term debt repayment will weaken the positive relationship between the two. The object of this paper is no longer limited to a single enterprise and a single institutional investor, but focuses on the whole portfolio, expanding the research on the influencing factors of institutional investors and audit fees.