Abstract: On July 18, 2021, a rare heavy rainfall event (referred to as “21.7” extreme rainstorm process) occurred in the northeast of Inner Mongolia. The maximum daily rainfall reached 279.7 mm, breaking the historical record in Hulunbuir City and causing serious disasters. Based on automatic weather station observation data, Doppler weather radar data, FY−4A satellite data and ERA5 reanalysis data (0.25°×0.25°) of ECMWF, the characteristics of the rainstorm process and the causes of its extremes were diagnosed and analyzed. The results are as follows. (1) The "21.7" extreme rainstorm process occurred under the influence of a low trough, the east-blocking situation of the subtropical high stretching northward and the high pressure ridge over the Middle Siberian Plateau, and the activity of Typhoon In-Fa (No.2106) over the western Pacific Ocean. It has the characteristics of wide influence, concentrated rainfall periods, extreme intensity and strong disaster. (2) The northward movement of the low-level southwest jet stream forms a strong differential pseudo-equivalent potential temperature advection, which leads to the unstable development of convective stratification. It also forms strong vertical wind shear with the near-ground easterly wind, promoting the organization of convective cloud clusters. The continuously triggered MβCS cloud clusters on the southern side of the low-level shear line and the northern side of the surface mesoscale convergence line, and their development along the guiding airflow, are the main reasons for the formation of the local extreme rainstorm. (3) Short term heavy rainfall events occur in the cold cloud region of mesoscale convective cloud clusters, where the TBB is less than 218 K and moves out of the edge of the azimuth gradient. The radar characteristics show that the strong convective cell is located on the upwind side of the convective cloud cluster identified from the satellite cloud image, suggesting a focus for monitoring and predicting extreme rainfall events using satellite and radar products. (4) Compared with the "19.8" historical extreme case, it is found that the "21.7" extreme process is rarer and more unique. The water vapor, energy and the duration of weather systems result in differences in extreme levels. During the "21.7" process, two water vapor conveyor belts strengthen northward through the Bohai Sea and converge in the northeast of Inner Mongolia, and the weather system moves slowly due to the situation of downstream large longitude east blocking.