Taking the merger of accounting firms in China Securities Audit Market in 2009 as case study, selecting the related data of listed corporations from 2007 to 2012, this paper makes an empirical research on the relationship among merger of accounting firms, signature tenure of CPA and audit quality. The research finds out that after the merging of accounting firms, the audit quality has significantly improved and the merged signature tenure of CPA in listed corporations has been positively related to audit quality. The results show that the size and quality of firms merged are obviously improved. Firms merged should take full account of the professional competency of the new signed CPA in the initial stage of the merger and the longer signature tenure of CPA, which are more conducive to the improving of audit quality.