In this paper, we use meteorological observations with high temporal and spatial variability [Fengyun satellite data (FY-2H), GPS/Met atmospheric precipitable water volume (PWV) data, and ERA5 reanalysis data]to characterize the large-scale circulation background field and mesoscale convective system (MCS) of an extreme rainstorm process on the northern slope of the Kunlun Mountains from June 14 to 17, 2021. The results showed the following. (1) The rainstorm process was characterized by many rainstorm stations and large cumulative precipitation, both localized and extreme. There were three extreme rainstorm centers in the Hotan Prefecture, where short-term heavy precipitation and continuous precipitation occurred, respectively. The duration of the short-term heavy precipitation process was short, with a maximum hourly rainfall of 29.4 mm. Meanwhile, the duration of continuous precipitation was 3 days, with an hourly rainfall of less than 5 mm. The upper troposphere was maintained by a two-body South Asian high pressure, and the formation and development of the Central Asian low vortex was a favorable circulation background for the occurrence of this extreme rainstorm. Under the joint action of high- and low-altitude rapids, strong dispersion in the upper level and convergence in the lower level promoted the development of vertical movement of the atmosphere. Moreover, a 500-hPa southerly flow, a 700-hPa shear line, and an 850-hPa easterly flow jointly provide a favorable power configuration for the rainstorm. (2) The water vapor transport in the middle troposphere was dominated by a southwest path and a southwest+south path, whereas that in the lower layer was dominated by a low-level easterly jet stream. The coupling of the water vapor transport paths in the middle and lower layers promoted the development and strengthening of this extreme rainstorm. The continuous moisture transport and strong water vapor flux convergence center before the occurrence of the extreme rainstorm made the atmospheric PWV in the rainstorm area show a significant humidification and gathering process before the precipitation, with the PWV reaching 30 mm. (3) Train-effect-type+merging- and intensifying-type MCSs were continuously generated over the rainstorm station and moved through, attributed to the direct influence system triggering short-term heavy precipitation, and the station was located at the maximum of the TBB gradient of the MCSs. The development and maintenance of meso-β- and meso-α-scale MCSs and the continuous coverage of vortex-like mesoscale convective cloud bands were the key systems that led to the occurrence of persistent precipitation at the storm site.