The high-precision precipitation fusion products CMORPH and CMFD were used as model inputs, and 18 flood events of the Tunxi Watershed in the upper reaches of the Xin’an River Basin from 2013 to 2018 were taken as the research object. The temporal and spatial distribution characteristics of rainfall by the two precipitation fusion products in the Tunxi Watershed were analyzed, and the applicability of the WRF-Hydro model driven by two types of precipitation fusion data to the simulation of mountain torrent processes in medium- and small-sized watersheds was explored. The results show that compared with the measured precipitation at the site, the CMORPH precipitation product has higher accuracy in terms of peak precipitation values and precipitation amount, while the CMFD product generally overestimates the precipitation amount, especially the peak precipitation values. At the same time, CMORPH precipitation product has higher accuracy on different time scales. In the simulation results based on CMORPH precipitation fusion data, only the Nash coefficient of three flood simulation results is less than 0.8 (one of which is less than 0.7), and the total qualified rate is 94%, reaching the grade A accuracy. The total qualified rate of flood simulation based on CMFD precipitation fusion data is 67%, which is grade C accuracy. Moreover, the model’s simulation accuracy for large floods is better than that of small flood processes. In general, the WRF-Hydro model driven by high-resolution CMORPH precipitation fusion product can be well applied to mountain torrent process simulation in medium- and small-sized watersheds.