If enterprises want to make M&A successful in the fierce market, effective internal control is one of the necessary conditions.This paper combines with five elements of the American’s COSO report on internal control of internal environment, which refer to the internal environment, the risk assessment, control activities, information and communication, internal supervision.This paper builds internal control Evaluation System based on samples of 56 buyout firms on the Shenzhen stock exchange A-shares happened in 2012 . Using the entropy weight assignment method to determine each factors weight in the internal control evaluation system, and this method helps to establish a more objective and reliable internal control evaluation system of the M&A.