Based on conventional observation data, intensive observation data at automatic weather stations, the optimal path of tropical cyclone from China Meteorological Administration, analysis fields from NCEP/GFS and black body temperature data from FY-2E satellite, the causes of heavy rainfall enhancement in Northeastern Zhejiang Province from 14 to 16 October 2017 caused by typhoon Khanun landing in Guangdong Province were analyzed by using weather diagnosis and numerical simulation method. The results show that the southeast jet and northeast wind in lower level formed typhoon inverted trough during the rainstorm. With the penetration of cold air on the ground, the frontogenesis formed in the coastal areas of Zhejiang Province, which was conducive to the formation of cyclonic circulation on the top of low-level inverted trough, and further led to the enhancement of heavy rainfall. The frontogenesis area was corresponding well with heavy rainfall region, and the horizontal divergence term was main contribution of frontogenesis process. At the beginning of precipitation formation, the large-scale circulation was beneficial to the formation of rainstorm, and the local topography and low-level convergence led to the occurrence of strong rainfall, but the convective instability wasn’t obvious. During the enhancement of heavy rainfall, the enhancement of convergence and convective instability in low level played important role in enhancing of precipitation. The enhancement of convergence in low level came from the formation and development of cyclonic circulation. The strong precipitation area was always located in the northeast side of cyclonic circulation, and the heavy rainfall area moved to northeast with the movement of cyclonic circulation to northeast.