Abstract: On July 18, 2021, there was an rare heavy rainfall event (referred to as “21.7” extreme rainstorm process) in the northeast of Inner Mongolia, the maximum daily rainfall reached 279.7 mm, creating the largest daily rainfall ever recorded in Hulunbuir, causing serious disasters. Based on automatic weather station observation data, Doppler weather radar data, FY−4A satellite data and ERA5 reanalysis data 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 background of the influence of the low trough, the east-blocking situation of the massive north of the subtropical high and the high pressure ridge over the Middle Siberian Plateau, and the activity of Typhoon Fireworks (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 is mainly due to the formation of strong differential pseudo-equivalent potential temperature advection, which leads to the unstable development of convective stratification. Secondly, it forms vertical wind shear with the near-ground easterly wind, promoting the formation of convective cloud clusters, and continuously triggered MβCS to the south of the low-level shear line and the north of the surface mesoscale convergence line, and generated and dissipated along the guiding airflow, which is the main reason for the formation of 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, and radar features show that the strong individual is located in the upwind direction of the convective cloud cluster on the satellite cloud image, which can be used as a focus for monitoring and predicting extreme rainfall events using satellite and radar products.(4) Compared with the historical similar extreme case of "19.8", it is found that the extreme process of "21.7" is more rare and special, the water vapor, energy and system duration constitute the difference in extreme degree. This process is manifested in that two water vapor conveyor belts strengthen northward through the Bohai Sea and converging in the northeast of Inner Mongolia, and the weather system moves slowly due to the situation of east blocking with large longitude in the downstream.