This paper looks at how online sales by enterprises affect audit fees. We use all A share listed companies in China from 2009 to 2020 to build a measurement index for enterprises that do online sales using the Python language. Through empirical testing, we found that enterprises engaged in online sales will significantly increase audit fees. This conclusion continues after a series of ruggedness tests. Mechanically, online sales increase audit risk, resulting in auditors taking more action, thereby increasing audit costs. Furthermore, the heterogeneity analysis shows that the "Big Four" firms, auditors with industry expertise and higher levels of information technology proficiency can weaken the positive impact of online sales on audit fees. In summary, we have clarified the impact and mechanism of online sales as a business model innovation on the audit fees of listed companies. Our research deepens our understanding of the impact of business model innovation on corporate operations and audit behavior, and provides useful insights for promoting the transformation and development of the audit industry.