Rejection of the traditional corporate M&A theories which regard managers as rational, Based on Behavior Corporate Finance theory to studies the impact of managers' over-confidence on companies' mergers and acquisitions(M&A) performances from the group decision-making perspective. We make an empirical research taking Chinese listed and issuing A shares companies as our samples. Results show that: Managers are prevalence of over-confidence in group decision making, There are significantly negative correlation between managers' over-confidence and long-term M&A performances, Though the correlation between managers' over-confidence and short-term M&A performances are also negative, statistically not significant. Finally, Based on results of this analysis, Give some alternative initiatives to overcome or reduce the managers' decision deviant behavior.