Based on conventional observations, data of dense automatic weather stations, satellite cloud imagery, radar observations and NCEP (National Centers for Environmental Predictions) reanalysis data, a systematic diagnostic analysis is conducted to investigate the causes of the extreme precipitation in Ningbo induced by Typhoon Muifa (2212) before and after its landfall in Zhejiang. The results are as follows. (1) The typhoon exhibits a pronounced asymmetric distribution before and after its landfall in Zhejiang, characterized by vigorous convection and intense echoes in the northwest quadrant, while the convection weakens significantly and echoes contract on the eastern and southern sides. (2) Radar velocity fields reveal a typical vertical circulation pattern of “low-level convergence and upper-level divergence” on the western-northern sides of the typhoon, providing favorable dynamic lifting for the convective development. (3) The persistent easterly flow transports abundant water vapor to northeastern Zhejiang, resulting in a strong water vapor convergence center at 925 hPa over Ningbo, which coincides well with the observed heavy rainfall area. (4) Furthermore, a shallow intrusion of weak cold air from the west, guided by a westerly trough, penetrates the low levels of the typhoon’s western side. The cold air interacts with warm and moist air masses and a sharp temperature gradient is created, triggering baroclinic frontogenesis and enhancing atmospheric instability, thereby significantly increasing the precipitation efficiency.