Existing studies have focused on the relationship between financial report inquiry letters and audit behavior, and the impact of non-financial report inquiry letters on audit behavior is unknown. This paper takes A-share listed companies from 2015—2021 as the research object to explore the impact of non-financial report inquiry letters on audit opinions. It is found that the more non-financial report inquiry letters received by listed companies, the higher the probability of being issued a non-standard audit opinion, and non-financial report inquiry letters are more conducive to the formation of objective audit opinions. The mechanism test suggests that non-financial report inquiry letters influence the auditor's audit opinion decision by revealing the business risk of the enterprise. Heterogeneity analysis reveals that the positive effect of non-financial report inquiry letters on non-standard audit opinions is more significant in the samples with low quality of internal control, low quality of disclosure, and no overstaffing of auditors. This study takes the type of inquiry letter as an entry point to deepen the research on regulatory inquiry letters and auditing behavior, and also provides new ideas for exploring the construction of a “regulator-intermediary” collaborative regulatory mechanism.