Based on the sample of all A share non financial listed companies in China from 2008 to 2018, this paper examines the impact of shared audit on audit quality. It is found that sharing audit with major suppliers or customers can significantly improve audit quality. Further analysis shows that in the horizontal sharing degree, the effect of shared audit mainly comes from the type of “shared audit firm and auditors”; the more the number of Certified Public Accountants sharing signatures, and the more sharing of lead partner and concurring auditor, the improvement of audit quality is more obvious. In terms of vertical sharing degree, compared with the companies without sharing audit, the companies sharing audit with customers and sharing audit with customers and suppliers at the same time have more obvious effect on improving audit quality. However, this paper does not find the evidence of higher audit quality of companies that sharing audit with suppliers, which indicates that the role of sharing audit in improving audit quality is mainly achieved through sharing audit with downstream enterprises. From the perspective of supply chain, the conclusion of this paper provides policy suggestions for improving audit quality and reasonable selection of auditors among enterprises, their suppliers and customers.