Exploiting the differential institutional incentives in the multi-level capital market, we study the impact of auditor joint practice (conducting annual audits at both markets in the same year) on audit opinion decisions in the NEEQ market. Using NEEQ-quoted firms from 2014 to 2019 as a research sample, we find that firms audited by joint practice auditors are less likely to be issued modified audit opinions than their counterparts, and mechanism analyses demonstrate that joint practice exerts influence through lowering audit effort, i. e. longer audit lag, lower information communication efficiency, and weaker industry specialization training. We conduct further analyses on auditor competence, the effect of joint practice on listed firms. Overall, this study provides empirical evidence for auditors'strategic favoring of one market over another in different institutional environments. Our findings have both theoretical and policy implications.