The existence of faultline forms different sub-groups easily, and the interaction between sub-groups will reverse and interfere the supervisory capacity of board, thus affecting audit quality. The impact of the board faultline on audit quality was explored, using the data of Chinese A-share manufacturing listed companies in Shanghai and Shenzhen from 2013 to 2021. The findings are as follows. First of all, the relationship between the board faultline and audit quality is an inverted U-shaped one. If the board faultline is too low or too high, it will interfere the normal performance of the oversight function of the board, thereby affecting audit quality. Secondly, the inverted U-shaped relationship between the board faultline and audit quality mainly originates from the role of the task-oriented board faultline, which is more significant in the mature enterprises. Thirdly, Internal control can weaken the impact of the board faultline on audit quality, while better internal control can help ease the agency costs between shareholders and management, and make up for the negative impact caused by the conflict between the sub-groups of the board, improve audit quality within a reasonable range.