The new audit reporting standards add the requirement to disclose key audit matters. This paper takes the text data of key audit matters from 2016 to 2020 as the research object, investigates the influence of auditor-client mismatches on the disclosure of key audit matters. It is found that the upward auditor-client mismatches significantly improves the readability, elaborate degree and quantity of key audit matters, thus helping to optimize the information environment of capital market.The downward auditor-client mismatches significantly reduces the readability, elaborate degree and quantity of key audit matters, which is not conducive to the improvement of information environment of capital market. Further analyses show that in the upward auditor-client mismatches, auditors are more inclined to explain the disclosure reasons of key audit matters, give conclusive evaluations, and disclose more industry-common risk matters and firm-specific risk matters. The research of this paper can provide useful reference for continuously improving the disclosure system of key audit matters and optimizing the information environment of capital market.