Abstract: Using multi-source meteorological observation data and NECP reanalysis data, an extreme rainstorm process in southwest Guangxi on May 17, 2020 was analyzed, The results show that the extreme rainstorm was produced by the joint influence of large-scale systems such as the high trough, the low-level shear line and the front. The development of the warm and low pressure on the ground in the early stage provided sufficient unstable energy conditions, and the strengthening of the southwest monsoon continuously transported water vapor from the Bay of Bengal and the northern South China Sea to Guangxi. The surface convergence line was an important trigger and maintenance mechanism of the extreme rainstorm process, and the special terrain in southwest Guangxi played an important role in the formation and movement of the convergence line. The convective echo had obvious backward propagation characteristics and formed a “train effect” during its movement. At the same time, the centroid of the precipitation echo was low and the warm cloud layer was deep, which had a very high precipitation efficiency and was the direct cause of extreme precipitation. The strengthening of vortex shear line and nocturnal boundary layer jet provided favorable conditions for the development of heavy rainfall, making the boundary layer always maintain strong water vapor convergence and lifting conditions. Meanwhile, the transport of warm and humid advection in the lower layer was also conducive to the maintenance of convective instability, providing a guarantee for the formation of extreme rainstorm.