Based on the sample of China’s A-share listed companies from 2003 to 2013, and from the perspective of the allocation of audit resources and audit opinion, the paper finds that audit firms have assigned more experienced auditors to areas with a good legal environment, but the results of non-standard audit opinion isn’t significant. Considering the audit firms of different sizes, there may be different sensitivity to the legal environment changes, and it may affect the final conclusion. This paper further investigates whether the audit firm size can strengthen legal environment’s promotion of high quality auditing supply effect. After considering the quality of auditors, we find BIG10 have assigned more experienced auditors to areas with a good legal environment, and the probability of issuing non-standard audit opinion is significant higher. This suggests that the auditor allocation has strengthened the legal environment’s promotion effect of high quality auditing supply. It also can be seen that BIG10 actually allocate audit resources according to different legal environment and provide different audit quality in different areas. Conclusions of this paper expand and complement the existing literature on the impact of legal environment on auditing supply, and have shown a positive policy implication for the allocation of audit resources and government regulation.