PurposeNoncontrolling large shareholders can reduce the agency problem of executives and can reduce the expropriation or tunneling behavior of controlling shareholders, thereby promoting corporate innovation. However, too many noncontrolling large shareholders may also lead to excessive supervision, thereby inhibiting innovative activities that contribute to the long-term value of the firm. Research to date, however, has not examined the nonlinear impact of noncontrolling large shareholders on corporate innovation. Based on principal–agent theory and the too-much-of-a-good-thing (TMGT) effect, the authors discuss the inverted U-shaped influence of noncontrolling large shareholders on corporate innovation and the moderating effect of industry competition and corporate product diversification on the above relationship.Design/methodology/approachBased on the empirical data of Chinese listed companies from 2003 to 2017, the authors use the bidirectional fixed effects model to conduct empirical testing and robustness testing of the research hypotheses.FindingsThere is an inverted U-shaped relationship between noncontrolling large shareholders and corporate innovation; type I and type II agency costs play a mediating role between noncontrolling large shareholders and corporate innovation. In addition, firm product diversification weakens the inverted U-shaped relationship between noncontrolling large shareholders and corporate innovation, but industry competition has no significant moderating effect on the above relationship.Practical implicationsThis research has important implications for policy makers, to better activate corporate innovation vitality, and investors, to better choose investment targets. Specifically, investors and policy makers should be aware that an appropriate increase in larger noncontrolling shareholders can maximize the enthusiasm of firms for innovation and enhance corporate value, but they should also realize that having too many noncontrolling large shareholders may backfire.Originality/valueThis research helps the authors to understand the pros and cons of increasing the number of noncontrolling large shareholders more comprehensively and also helps to understand corporate innovation more comprehensively from a supervisory perspective. In addition, this research also enhances the explanatory and predictive power of the TMGT effect.