The Tarim Basin in Xinjiang is a world-renowned arid area with an average annual precipitation of less than 100 mm. A rare rainstorm occurs in the basin around July 19, 2021. The maximum cumulative rainfall and daily rainfall are 107.3 mm and 78.5 mm (both up to the magnitude of Xinjiang’s heavy rainstorm). , the following conclusions are drawn by analyzing the characteristics of the storm water vapor. For the first time, the concept of the "uniform twin" of the South Asian High was proposed. During the transition of the 100 hPa South Asian High from the "high in the west to the 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. "Two highs and one low" circulation situation; reveals the large-scale circulation and physical mechanism of the water vapor from the Arabian Sea and the northern Bay of Bengal entering the basin under the stable anticyclonic circulation of Iran under high pressure. Clarify that 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, southwest + south, and pointed out that the easterly wind on the south side of Iranian high pressure and Central Asia 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 west, south, and east boundaries. The water vapor income from the east boundary mainly comes from the low-level east wind. The water vapor income from the west and south boundaries comes from three middle and high-level paths. The "southwest + south" path water vapor transport causes water vapor at the south boundary. The input contribution is significantly greater than the western boundary. The heavy rain in the Tarim Basin should not only pay attention to the Central Asian low pressure, but also pay attention to whether the Central Asian low pressure, the Indian low pressure, and the plateau vortex coexist and affect the atmospheric circulation field.