Abstract: In order to deepen our understanding of the development mechanism for mesoscale systems that are conductive to torrential rain in Xinjiang Uygur Autonomous Region, this paper analyzes a rainstorm process occurred over the Aksu region in the western Tianshan Mountains during 17-18 June 2013. Various data including the FNL analysis of the U.S. Environmental Prediction Center, the global reanalysis data from the European Center for Medium-range Weather Forecast, the ground automatic weather station data provided by the China Meteorological Administration, the satellite equivalent blackbody brightness temperature data and the WRF high-resolution numerical simulation data are used in the present study. The results show that the precipitation occurred under the background of synoptic circulation with "two-ridge and one-trough" pattern over the middle and high latitudes, and the Central Asian Vortex provided favorable synoptic-scale dynamical and water vapor conditions for this heavy rain event. The convergence line was the most important mesoscale system for this heavy rainfall event, which was formed by the superposition of the Central Asian Vortex circulation and the flow around the special topography of the western Tianshan Mountains. Due to the thermal contrast between the mountains and valley, the downslope wind drove the convergence line to move and strong convection developed along the convergence line, triggering heavy precipitation in Aksu region. The WRF model is able to well simulate not only the location and intensity of the heavy rain, but also the evolution of the wind field. Preliminary analysis of observations and simulation data show that due to the blocking effect of the western Tianshan Mountains, southerly winds converged near the valley. As a result, a local convergence line formed. Meanwhile, following the adjustment of large-scale circulation, especially after the Central Asian Vortex moved to the vicinity of Aksu area, part of the westerly flow in the south of the vortex crossed the western Tianshan Mountains and turned to northwesterly flow, while the remaining part of the westerly flow passed through the Yili Valley and turned to northeasterly winds. These two flows further aggravated northerly winds in Aksu region, and subsequently promoted the formation of mesoscale convergence line. Water vapor brought in by easterly winds in the east of the convergence line accumulated in front of the southern foothill of the Tianshan Mountains. As the valley wind enhanced in the nighttime, the convergence line moved southeastward and continued to intensify. Accompanied with the uplifting of airmass and accumulation of water vapor at the foothill of the mountains, unstable energy was released and heavy precipitation occurred in Aksu region.