Based on a research sample of A-share listed companies from 2007 to 2019, this paper empirically examines the impact of voluntary disclosure by companies of customers' information on companies' performance in the product market. The results indicate that the more detailed the information disclosed about customers, especially regarding customers' identities, the more significantly the company's product market return decreases in the following year. This effect is more pronounced among non-state-owned companies, companies in non-regulated industries, companies with greater R&D expenditure, companies in more competitive industries, follower companies, and companies not audited by big four audit firms. The effect is more pronounced when there is a listed customer or a related customer. Mechanism tests indicate that disclosure of customers' identities will undermine companies' competitive position, customer stability, pricing power and operating assets turnover rate. This research is useful for analyzing the policy effects of current customer information disclosure requirements and provides guidance for disclosure policies in emerging markets.