Abstract: In September 2020, typhoon Maysak experienced an extratropical transition. The numerical model WRF4.2 was used to simulate this event. The simulation results showed that as the typhoon interacted with a high-latitude cold trough, it resulted in heavy rainfall during its northward journey. This study investigated the thermodynamical, dynamical, and cloud microphysical characteristics of the typhoon’s low-pressure center and outer cloud area. Furthermore, it explored the effects and mechanisms of the cold trough on the spatial distribution and temporal variation of rainfall intensity. Results showed the following. (1) In the typhoon’s low-pressure center, the invasion of dry and cold air from the middle layer decreased the convection height. This created an unstable structure that was dry at the top and wet at the bottom, but the convection intensity was maintained in the middle and lower layers. Conversely, in the typhoon’s outer cloud area, the convection height remained stable owing to vertical vorticity transfer. Cold air invading from lower layers lifted warm air, enhancing the upward motion of the middle and upper layers. As a result, Maysak gradually evolved into a forward-leaning convective structure. (2) The cloud microphysical processes of this precipitation primarily involved the transformation of water vapor into snow through desublimation at the upper levels. As the snow fell, it collected cloud water and grew, eventually melting into rainwater within the melting layer and continuing to absorb additional cloud water. Simultaneously, a portion of the rainwater originated from the melting and rainwater collection of graupel. Graupel was produced as snow particles collected moisture from cloud water. (3)The two major sources of rainwater were the melting of snow and the collection of cloud water by rainwater. The cold trough affected the typhoon’s thermodynamic structure, affecting the spatial distribution of cloud water and snow and the spatial distribution of precipitation. In the low-pressure center, cloud water concentrated toward the core, while in the outer cloud area, snow was more prevalent ahead of the cloud water along the outflow direction. Rainwater was mainly distributed where snow and cloud water overlapped. Quantitatively, the intrusion of dry and cold air led to higher rates of snow desublimation and rainwater evaporation in the outer cloud area, while cloud water condensation efficiency was low. This resulted in a higher proportion of snow and a lower proportion of cloud water and rainwater in the precipitation particles. (4) The cold trough primarily affected the vertical velocity of the typhoon, directly impacting the efficiency of snow desublimation and cloud water condensation. This, in turn, influences the efficiency of snow collecting cloud water, converting into graupel, and melting to rainwater, as well as rainwater collecting cloud water and graupel melting to rainwater, ultimately leading to the change of precipitation patterns.