The key to balancing economic transformation and improving quality development is financial supporting green development. The relationship between financial technology (fintech) and green development has gradually emerged recently. Based on the data of 35 major cities in China from 2015 to 2019, the fintech development index and green total factor productivity (GTFP) are obtained by adopting web crawler technology and Bootstrap-SBM-GML model respectively; further, the impact of urban fintech level on GTFP is also revealed by taking the financial support policy index (FSP) as the instrumental variable. The empirical research shows the following results. First, the level of fintech and financial support policy indicators show a steady upward trend in the study period; whereas the tendency of GTFP is not obvious. Second, the urban fintech level has a significant promoting effect on GTFP through FE, MM-QR and 2SLS models. Specifically, the promoting effect mainly comes from the promotion of technological change (TC) of the GTFP decomposition index; the promoting effect will be greater in the lower cities of green development level. Third, industrial structure upgrading (UIS) and technological innovation (TI) play an intermediary role in the green development effect of fintech. Four, the green development effect of fintech is heterogeneous. Specifically, the green development effect of fintech on GTFP is larger in the central and western regions and low-level cities; whereas it is smaller in the eastern part regions and high-level cities.