Based on the GPM_3IMERGM precipitation data, automatic ground encryption station data, the Blackbody Brightness Temperature (TBB) and ERA5 reanalysis data, the variation characteristics and causes of the heavy rain belt before and after Typhoon "In-fa" landing in Zhejiang were studied. Results are as follows: (1) before and after the landing of Typhoon "In-fa", the rainfall intensity weakened slowly, and the distribution of the heavy rain belt was asymmetric. Before landing, the heavy rain center was in the north, and there was a strong rain belt outside. After landing, the heavy rain center shifted to the east, and a tail rain belt appeared. (2) Typhoon "In-fa" had weak guiding airflow and slow moving speed.Typhoon "Cempaka" and "Nepartak" strengthened the water vapor transport, so that the intensity of "In-fa" weakened slowly, and the strong rain belt remained all the time; before the landfall, the main water vapor transport was from the South China Sea, and after the landfall, the water vapor transport was mainly from the eastern West Pacific, which was conducive to the eastward migration of the rainfall center. (3) There was a good correspondence between the variation of vertical wind shear and the movement of rainband center of "In-fa", and the rainband center was generally located on the left or in the direction of shear. Before the typhoon landed, the vertical divergence field structure tilted to the north, and the ascending movement in the typhoon area weakened from north to south. After the typhoon landed, the divergence field structure tilted to the east, and the ascending movement weakened from east to west, and the typhoon rainfall center gradually shifted from the north to the east. (4) The outer rain belt of "In-fa" before landfall was affected by the convergence of Hangzhou Bay and the uplift of Longmen mountain, and the tail rain belt after landfall was mainly related to the friction convergence near the coastline and the uplift of Simin mountainous areas.