Does the Carbon Emissions Trading Pilot Policy Affect Auditor Industry Specialization? Evidence from China and Implications for Sustainability

China’s Carbon Emissions Trading (CET) Pilot Policy may alter corporate information environments and demand for auditors with industry-specific experience. Using the staggered launch of regional carbon markets as a quasi-natural experiment, this study examines whether policy exposure changes auditor industry specialization among Chinese A-share listed firms from 2010 to 2022. Specialization is measured by the logged share of an auditor’s client assets in a given industry; it does not represent an audit-fee premium or direct carbon-assurance expertise. Staggered difference-in-differences estimates show that CET pilot exposure increases this specialization measure. The result is robust to event-study, placebo, propensity-score-matched, and Callaway–Sant’Anna specifications. Channel regressions are consistent with financing-constraint and leverage pathways, while evidence for controlling-shareholder fund occupation is weak and not interpreted as causal mediation. Pairwise tests identify regional differences but no statistically significant ownership or pollution-intensity differences. By linking carbon-market policy to auditor-client matching, the study shows that sustainability policy can reshape the professional-service infrastructure supporting firms’ low-carbon transition. Allocating industry-experienced auditors to policy-exposed firms may strengthen the credibility and comparability of sustainability-related financial information and support regulatory monitoring. These implications concern the institutional capacity for sustainable development; the study does not directly measure sustainability performance or carbon-assurance quality.