Abstract: The Tarim Basin in Xinjiang is a world-renowned arid area with an average annual precipitation of less than 100 mm. A rare heavy rain occurred in the basin around July 19, 2021. The maximum cumulative rainfall and daily rainfall were 107.3 and 78.5 mm, respectively (both values are up to the magnitude of Xinjiang’s heavy rain). The following conclusions were drawn by analyzing the characteristics of the storm water vapor. For the first time, the concept of a “uniform twin” of the South Asia high was proposed. During the transition of the 100-hPa South Asian high from “high in the west to low in the east,” the 500-hPa Iranian high and the plateau anticyclone, the central Asian low and the Indian low, and the plateau vortex were jointly structured. The “two highs and one low” circulation situation reveals the large-scale circulation and physical mechanism of vapor from the Arabian Sea and the northern Bay of Bengal entering the basin under a stable anticyclonic circulation of Iran under high pressure. The main sources of storm water vapor in the basin are the Mediterranean Sea and the western ocean, central Asia, the Arabian Sea, and the Bay of Bengal. The water vapor transport has three paths and trajectories: west, east to west, and southwest + south, and the easterly wind on the south side of the Iranian high pressure and central Asian regional westerly winds play a key role in the “east to west” water vapor transport. The easterly winds in the Arabian Sea and the Bay of Bengal merge with the west wind belt to form a water vapor conveyor belt that is an important condition for the occurrence of this heavy rain. Water vapor is input from the western, southern, and eastern boundaries. The water vapor income from the eastern boundary mainly comes from the low-level east wind. The water vapor income from the western and southern boundaries comes from three middle and high-level paths. The “southwest + south” path of water vapor transport brings water vapor to the southern boundary, where the input contribution is substantially greater than that of the western boundary. The heavy rain in the Tarim Basin depends on the central Asian low pressure and whether the central Asian low pressure, Indian low pressure, and plateau vortex coexist and affect the atmospheric circulation field.