In this paper, the organization process and mechanism of a rainstorm in the arid area of western southern Xinjiang are diagnosed and studied using the PV (potential vorticity) wave-activity density and wave-relation equation. The key physical processes affecting the organization of the rainstorm convection system are analyzed and discussed in detail. The PV wave-activity density couples a variety of atmospheric dynamics and thermal disturbances affecting the evolution of convective clouds, which can well describe the organization process of convective systems. On this basis, the wave-relation equation describing changes in the PV wave activities can be used to study the physical factors driving the organized development of convective systems. Many physical processes diagnosed by the wave-relation equation are related to the development and evolution of disturbed baroclinic, disturbed wind shear, and disturbed vorticity, indicating their important role in the convective organization. During the organization process, multiple east–west oriented convective lines develop into a northeast–southwest banded convective system, including the maintenance of strong convection and the increase in north and south scales. The development and enhancement of the weak convection in the northeast direction of the convective line are related to the enhancement of the baroclinic disturbance caused by the thermal transport of basic state flow from the strong convection area to the weak convection area. The key processes affecting the maintenance of the strong convection in the middle of the convective line include the thermal transport caused by updraft and downdraft, which leads to the enhancement of baroclinic disturbance in the convective line; the cyclonic circulation formed by disturbed west and east winds, which leads to the enhancement of meridional shear circulation; the disturbed meridional wind transport of the disturbed zonal wind shear to the convective center, resulting in the enhancement of zonal shear and vertical shear circulation. The study shows that the organization of the convection system results from the evolution and cooperation of various dynamic and thermal disturbances in the atmosphere. The key process can be clearly reflected through the wave-relation equation and predictive significance, for which the organization of the convection system can be discussed in the future.