The irrational behaviors of managers, that is, “overconfidence”, which may not only affects diversification strategy, but also exacerbates corporate credit risk. In this paper, from the angle of behavior science, on the relationship between managerial overconfidence and corporate diversification and its impact on corporate credit risk has been discussed, furthermore, empirical examination on the Chinese listed companies over the period from 2009 to 2012. The study found: Managerial overconfidence significantly affects the company's diversified operations. Company’s diversified operations have mediating effect between managerial overconfidence and corporate credit risk. In addition, diversification of the prior period can significantly positive affect the manager's overconfidence, while the effects of credit risk of the prior period on the psychological impact of the overconfident managers were not significant. The results of this study have great reference value to the managers’ behavior modification and enterprise management strategies’ formulation.