By using the data of A-share listed companies in China from of 2012 to 2014, taking discretionary accruals as proxy of audit quality and the number of registered accountants in the office as the measurement of office size, this paper makes an analysis on the relationship between audit quality and office size, and the moderating effect of reputation mechanism on firm size and audit quality．Empirical studies have found that the size of branch and audit quality is positively related. Namely, the greater scale of the branch can further inhibit the earnings manipulation, thus audit quality is improved. In addition, the reputation protection mechanism of the firm has a reverse effect on the relationship between firm size and audit quality. Compared with non-“Big Four” accounting firms, the positive relationship between firm size and audit quality is becoming weaker in the “Big Four” branches.