This paper empirically examines the effect of market structure concentration on abnormal audit fees and further investigates the moderating effect of local protection on audit contracts, using listed companies in Shenzhen and Shanghai from 2015-2022 as the research sample. This paper finds the following: First,market structure concentration has a significant negative effect on abnormal audit fees, and local protection further enhances the negative relationship.Second, the effects of market structure concentration and local protection on abnormal audit fees are more significant when the abnormal audit fees are negative, and the utility of external supervision of the annual report inquiry letter is constrained by local protection.Third, market structure concentration affects abnormal audit fees through agency costs and audit length, which in turn impairs the robustness and comparability of accounting information.This paper not only enriches the research related to market structure concentration and CPA audit, but also provides ideas that can be applied to transform local government functions, regulate low-cost audit competition and improve corporate governance.