Typhoon “Ambi” was the first tropical cyclone to enter Inner Mongolia, causing rare catastrophic heavy rainstorm in the central and eastern parts of the region. This paper analyzes the transformation mechanism of “Ambi” during its northward movement and its impact on the heavy rainfall weather in Inner Mongolia using simulation results from the mesoscale numerical forecasting model (Weather Research Forecast, WRF), reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), typhoon path data, and routine observational data. The results indicate that the large-scale atmospheric circulation background of the heavy rainstorm was the interaction between the typhoon and the mid-to-high latitude westerly trough, where the cold air provided by the westerly trough met with the warm and moist airflow of the typhoon, triggering this heavy rainfall event. During the process of the westerly trough merging with the typhoon, cold air intruded from west to east and moved downward, generating a strong cold advection that lifted the warm and moist air, disrupted the barotropic warm core structure of the typhoon, and formed an asymmetric temperature structure of “warm in the east and cold in the west”, transforming the typhoon into an extratropical cyclone. The main precipitation area was located in the overlapping zone of positive MPV1 and negative MPV2 values, where the accumulation of convective instability and baroclinic instability energy promoted the development of intense precipitation. In the region where cold and warm air intersected, strong frontogenesis occurred, forming a distinct frontal zone, which provided the dynamical conditions for heavy rainstorm. Near the frontal band, a noticeable secondary circulation developed, with ascending airflow at the front and descending airflow at the rear, with the area of strongest precipitation corresponding to the region of strongest ascending airflow.