Abstract: The effect of radar data assimilation under different horizontal resolutions on the numerical simulation of a squall line that occurred in Chongqing on 5 June 2019 was studied in this paper. The Weather Research and Forecasting (WRF) model and the three-dimensional variational assimilation (3DVAR) system of the advanced regional prediction system (ARPS) and its data assimilation system (ADAS) were used. Our major conclusions are as follows: (1) Without radar data assimilation, the simulation results are still not significantly improved with the horizontal resolution increasing from 900 m to 300 m. (2) In comparison, with radar assimilation, the simulations under different horizontal resolutions have a certain improvement in the shape, intensity and falling area of the radar echo. (3) Combining an adequate higher resolution with the initial radar data assimilation can further optimize and adjust the dynamic thermal water vapor conditions of the simulation, making the simulated characteristics of evolution process and organizational structure of this squall line closer to the observations. Therefore, improving the horizontal resolution and using radar data assimilation at the same time might significantly improve the model prediction capability.