During June 14th—17th, 2021, an extreme rainstorm process with a daily maximum of 106.6 mm occurred over the Tarim Basin. This study analyzes the synergistic mechanism between the westerly wind and Indian monsoon on this rainstorm process using precipitation data from an automatic meteorological station in the basin, the GRAPES-GFS analysis field, and ERA-5 (0.25° × 0.25° , hourly) reanalysis data, as well as the WRF-v4.2. 2 numerical simulation model and the HYSPLIT-v4.0 water vapor backward trajectory model. Under the circulation background of “high geopotential height on the east side and low on the west side” at 100 hPa and “one low trough between two high-pressure ridges” at 500 hPa, water vapor is transported from Black Sea, Caspian Sea, Salt Lake, northern Indian Ocean, Central Asia and northern Xinjiang, corresponding to westerly, southerly, and easterly water vapor tracks, each transported along 850—300 hPa, 500—400 hPa, and below 650 hPa. The Indian monsoon circulation plays a key role in transporting water vapor from the Indian Ocean to the Tarim Basin, elucidating the physical process of the southward movement of the Indian monsoon carrying water vapor into the basin. The disappearance of the “south wind window” water vapor transport is an important manifestation of the synergistic influence of westerly winds and the Indian monsoon.