In recent years, extreme precipitation events have occurred frequently. Because of their short duration and complex mechanisms, they are challenging to predict accurately. Cities, as hubs of human production and life, have exacerbated the intensity and frequency of extreme precipitation events due to changes in their underlying surfaces. To investigate the impact of urban underlying surfaces on heavy precipitation, this paper takes a typical heavy precipitation event in Beijing as a case study, using the mesoscale numerical model WRF for simulation and analysis. Based on the outcomes of the simulations, sensitivity tests were conducted on the urban underlying surface in Beijing's urban area. Results indicate that: (1) during this severe convective event in Beijing, the upward airflow ahead of the trough and sufficient moisture transport provided favorable conditions for the precipitation process. (2) The urban underlying surfaces caused an average increase in surface temperature by more than 4 K within the city, resulting in an augmentation of over 26 W·m^-2 in sensible heat flux. This heat was then transported from the ground to the lower atmosphere in the form of sensible heat, making the previously neutral and conventionally stable atmospheric conditions unstable. (3) The urban heat island effect enhances vertical motion in urban areas, prolonging the duration of precipitation and increasing the central value of extreme precipitation. Furthermore, this enhanced vertical motion also propagates downstream, leading to an average increase in precipitation of less than 1.1 mm·h^-1 in the downwind area. Urban expansion expands the range of the water vapor convergence zone, thereby further intensifying the convergence of moisture and facilitating precipitation.