In order to reveal the evolution characteristics and effects of low-level jet (LLJ) during the formation of summer rainstorms, comprehensive observational data and reanalysis data are used to compare and analyze the two summer rainstorm cases with different synoptic types and rainstorm intensities in Beijing. The data include the wind profiler radar (WPR)， NCEP/GFS reanalysis data (0.5°×0.5°), and hourly precipitation data from automatic weather stations. The results show that the 16 July rainstorm process in 2018 (shortened as “7·16” rainstorm) was formed under the influence of southwest low-level jet (LLJ) at the edge of the subtropical high. The eastward movement of the vortex and trough, the continuously strengthening of the LLJ, and the influence of terrain convergence line provided extremely favorable conditions including low-level water vapor transport, convergence and uplift conditions for the occurrence of extreme rainstorm. The 22 July rainstorm process in 2019 (shortened as “7·22” rainstorm) was formed under the influence of the LLJ in front of the upper-level trough, which was caused by downward propagation of the upper-level jet during its eastward movement. This kind of LLJ has relatively smaller impacts on water vapor transport and convergence in the lower layers. Within 3 hours before the two rainstorms, similar characteristics appeared as follows: the low-level wind speed increased, the minimum height of LLJ decreased, the LLJ index sharply increased, obvious vertical wind shear occurred below 1500 m, and the vertical wind shear gradually〖JP2〗 increased with the approach of rainstorm. During the “7·16”〖JP〗 rainstorm, the occurrence of the convective cells was triggered by the boundary layer jet (BLJ), and the developing convective system was well organized by the joint action of the LLJ and the terrain convergence line. So, these are the key reasons for the formation of extreme heavy rainfall along the mountains. The characteristic parameters of the LLJ (maximum wind speed of LLJ, minimum height of LLJ, LLJ index) and the magnitude of the vertical wind shear below 1500 m are important indicators for the occurrence of rainstorm.