The extreme rainstorm process in Beijing from July 15 to 16, 2018 was analyzed by using the data of automatic ground stations, radar and NCEP and the standardized anomaly method. The results show that: the extreme precipitation occurs between the subtropical high and the high trough, with obvious southwest jet in the lower layer and abundant water vapor; when the rainstorm occurs, the standardized anomaly of environmental conditions exceeds 3 σ, which created the favorable dynamic, water vapor and unstable conditions for this extreme precipitation; meso-scale convective system activities can be divided into two stages,the convective system triggering and local heavy rainfall, and the convective system organization strengthening and northward movement of heavy rainfall area, both of which are accompanied by obvious train effect with very high precipitation efficiency; the low-level southwest jet is conducive to the establishment of atmospheric unstable stratification and the continuous transportation of water vapor. Meanwhile, the direction of the ground convergence line is consistent with the direction of the 850 hPa low-level jet and the movement direction of convective system, which is the main reason for the enhancement after the triggering of the convective system.