From August 10th to 11th, 2020, there was a regional heavy rainstorm in Ya'an City. Among them, Lushan County saw a heavy rainstorm. The heavy rainstorm lasted a long time, and the maximum accumulated precipitation and maximum hourly rainfall were relatively large. All have broken through historical extremes and have strong extremes. Through the analysis of radar echo, it can be known that the extreme heavy precipitation was caused by the stable and low movement of the heavy precipitation supercell storm. This article is based on the rainfall data of 365 regional automatic stations in Ya'an City, micaps live data, radar, and NCEP 1°×1°. Analyze data and other data to analyze the reasons for its occurrence: (1) The planetary scale and weather scale systems are stable and less moving, and the low-level night and low-level jets develop and transport warm and wet advections, which are conducive to the establishment of unstable convective stratification, which is a super unit of heavy precipitation. The occurrence of physical storms provides favorable circulation conditions. (2) The pseudo-equivalent potential temperature from the ground to the lower layer is abnormally high, which is conducive to the further development of convective instability. There are also dense areas of meso-scale pseudo-equivalent potential temperature near the strong convective storm. The level circulation is conducive to the transport of unstable energy and water vapor from the lower layer to the upper layer, and it also triggers the release of unstable energy, resulting in a strong upward movement. (3) Strong vertical wind shear and large convective effective potential are beneficial to the occurrence of heavy rainfall supercell storms and the maintenance of their organized degree. (4) Abnormal water vapor conditions are also an important reason for the heavy rainfall supercell storm. (5) Under extreme high-energy and high-humidity atmospheric conditions, the mesoscale convergence line of the boundary layer triggered the direct cause of the heavy precipitation supercell storm, and the heavy precipitation supercell is stable and less moving due to the blocking effect of the terrain. This resulted in the occurrence of this extremely heavy precipitation.