Abstract: The mechanism affecting audit quality by suchsocial capital as information, knowledge, and reputation that are contained in the social network relationships of auditors has received widespread attention from the academic community. This paper conducts an empirical study based on the A-share private listed companies from 2010 to 2020. The findings show that the richer the social capital of auditors, the higher the audit quality. Further testing reveals that external regulation can significantly enhance the positive correlation between auditors’ social capital and audit quality, that the social capital of auditors can significantly reduce the probability of the occurrence of fraudulent financial restatements, but has no significant impact on the non fraudulent financial restatements, and thatcompared to political social capital, commercial social capital has a more significant impact on the improvement of audit quality. In addition, when the auditor’s social capital is more than the client’s social capital, the auditor’s social capital can better improve audit quality. Therefore, auditors should strengthen their accumulation of social capital; private listed companies should pay attention to the social capital of auditors when selecting accounting firms; the regulatory authorities should pay attention to the matching situation between auditors and clients to avoid damaging audit independence due to clients having a say in the audit business.

Key words: social capital, social network relationship, audit quality, private listed companies