Optimizing the market structure of banking plays an important role in promoting economic development and alleviating financing constraints of SMEs.Based on the banking，economic and financial data of 48 counties （cities） in Jiangsu province from 2014 to 2019，the fixed effect model is used to analyze the impact of fintech on the market structure of county-level banking.The results show that the development of fintech will significantly reduce the market concentration of county-level banking，and this conclusion remains valid after considering possible endogeneity issues and a series of robustness tests.Further research shows that the reason for the decline of the market concentration of the banking is that the development of fintech reduces the share of rural commercial banks in the county-level financial market.The results of heterogeneity analysis show that development of fintech plays a more significant role in reducing the market concentration of county-level banking for counties with high level of economic development，low level of government intervention and high degree of economic openness.Accordingly，the paper puts forward policy suggestions on developing fintech to optimize the market structure of county-level banking.