This paper selects the merger case of Shine Wing in 2005 and the allied case of BDO China SHU LUN PAN Certified Public Accountants in 2006 as our empirical research cases. We use the value of DA calculated by the Jones model as substitution variables of audit quality to study whether audit quality has changed significantly after the expansion. Firstly, we find that the audit quality of Shine Wing is obviously enhanced overall after the merger, especially the audit quality of its domestic firm has improved more significantly than its firm in Hong Kong; secondly, the members of SHU LUN PAN increase by means of brand alliance, although the audit quality of its small members has improved, that of SHU LUN PAN and the alliance has gone down significantly. This research has shown that the further consolidation after accounting firms expansion is the key to improving the quality of audit.