Typhoon Hagupit (No.04,2020) caused torrential rainfall in the mountain region as it passed over North Yandang Mountain after making landfall in the south coastal areas of Zhejiang Province. Based on the Weather Research and Forecast (WRF) version 4.0.2 mesoscale numerical model, a high-resolution numerical simulation is conducted on typhoon Hagupit to analyse the effect of North Yandang Mountain on the occurrence of this heavy rainstorm caused by the typhoon. Sensitivity experiments, which involve adjusting the terrain height by lifting or reducing it, are carried out to investigate the role of terrain. The results are as follows: The track and intensity of the typhoon were well simulated by the numerical experiment over time. The simulated torrential rain triggered by the typhoon was consistent with the observation, including the rainfall area and intensity. The distribution of the strong wind centre caused by typhoon Hagupit was noticeably asymmetric. The first and fourth quadrants of typhoon Hagupit successively passed over the mountain region after the typhoon made landfall, which transported sufficient water vapour to the mountain region through a strong southerly jet on the east side of the typhoon. A long and narrow spiralling convergence band was observed at low altitudes above the mountain region, which was brought from the inner core of typhoon Hagupit before the landfall. The location and distribution of the water vapour flux convergence band were consistent with the wind convergence band. Strong upward motion occurred on the windward slope along the heavy rain centres when the eyewall of the typhoon passed over, indicating favourable dynamic conditions for the formation of torrential rain. The water vapour transport band extended upward from the near surface into the lower atmosphere, and scattered convective cells started to be triggered and enhance in the mountain region simultaneously. As the back of typhoon Hagupit passed over, the winds weakened in the high altitude area of the mountain region, resulting in flow around and the formation of mesoscale vortices, which contributed to the heavy rainstorm. The primary reason for the appearance of torrential rainfall was the presence of a strong, persistent, and fixed positive vorticity centre on the windward slope of the heavy rain centre. In sensitive experiments involving the terrain height, it was observed that the accumulated precipitation decreased by 40% to 50% when the terrain height in North Yandang Mountain was set to zero, while it increased by over 60% when the terrain height was doubled in the mountain region.