On August 13-14，2022，an extreme rainstorm event occurred in Yuzhong region of Gansu Province， northeast of the Qinghai-Xizang Plateau. Accumulated daily precipitation reached 130. 6 mm and the maximum hourly precipitation was 36. 6 mm，breaking the heaviest daily precipitation records of the region and causing serious social impact and economic losses. Based on the data of surface minute observation and high altitude observation，Lanzhou Doppler radar and ECMWF Reanalysis v5（ERA5），by analyzing the observation characteristics，environmental conditions，topographic effects and instability mechanism of the two heavy precipitation stages in this extreme rainstorm，the results show that：（1）The rainstorm was caused by the convergence of weak cold air brought by the shortwave trough in the westerlies and warm and humid air outside the subtropical high in the Longzhong area. The 700 hPa shear line provided the dynamic lifting conditions，and the surface cold front provided the triggering conditions.（2）The radar reflectance factor in the rainstorm process was characterized by persistent strong echoes accompanied by "backward propagation"，low-level jets and obvious convergence. In the second stage，the echo top height behind the cold front was similar to that in the first stage，but the scope was larger and the structure was more compact，and the convective cloud development was more vigorous. （3）The water vapor conditions of the rainstorm were abundant. In the first stage，there was significant convective instability due to strong convergence and upward movement at the lower level and high convective effective potential energy. In the second stage，the upward movement was weakened，the convective effective potential energy was 0，and the dynamic and convective instability conditions were weak.（4）The release of unstable energy triggered by cold front baroclinic frontogenesis was the main triggering mechanism of precipitation in the first stage. After the transit of the cold front，the precipitation in the second stage was formed by the combination of terrain，frontogenic secondary circulation and instability. Since the heavy precipitation after the summer cold front was not common in the northeast part of the Qinghai-Xizang Plateau，forecasters tended to ignore such kind of rainstorms. Therefore，we need to strengthen monitoring and early warning of such rainstorm events.