As the entity of providing audit service for the society, CPA firms internal management inevitably has a great effect on the audit quality. By probing into the “black box” of CPA firms, this paper investigates the effect of stuffs behavior mode and engagement quality control system of CPA firms on the audit quality. This research finds that the audit quality of the CPA firms in which there exists a defect in staffs behavior mode and engagement quality control system, i.e. there is a material misstatement in the audited financial statements, is systematically lower than that of other CPA firms. This paper refers to the phenomenon of a serious low quality audit predictive of systematic low audit quality as the contagion effect of audit quality of CPA firms.