In order to realize the high quality economic development and improve the scientific and technological innovation ability of enterprises, the research and development investment of listed enterprises in China has become increasingly concerned. Does R&D investment affect audit expenses? Based on the data of Chinese A-share listed companies from 2011 to 2020, this paper investigates how R&D investment affects audit fees. The results show that the higher the R&D investment, the higher the audit cost. Further research shows that firm size has a negative moderating effect on the impact of R&D investment on audit fees.