Based on automatic weather station observations of precipitation，ERA5 reanalysis data and NCEP GDAS data，through methods such as water vapor budget analysis，HYSPLIT backward trajectory tracking and water vapor transport contribution rate，this paper analyzed the water vapor transport characteristics of a severe rainstorm event that occurred in the southeastern part of Gansu Province from July 22 to 24，2024，and quantitatively discussed its water vapor source and contribution rate. The results showed that the rainstorm area was located in the exit zone of the upper-level jet stream，where the airflow divergence enhanced the vertical transport of low-level water vapor，causing the moist layer deeper. The continuous synergic influence of Typhoon " Prapiroon" and the subtropical high transported water vapor over the South China Sea and the Bay of Bengal to the rain‐ storm area，providing sufficient water vapor，and the rainstorm area maintained a significant near-surface wet area and high precipitable water. The results of water vapor budget and tracking analysis showed that the inflow of water vapor mainly occurred at the southern boundary of 800~500 hPa，with a maximum inflow of 1237 kg·m-·1 s-1. Before the rainstorm，the water vapor mainly originated from the South China Sea and the Bay of Bengal，and the water vapor channels accounted for 48% and 42% of all tracks respectively，and the water vapor transport contribution rate was 51. 45% and 43. 31% respectively. When the rainstorm occurred，water vapor mainly originated from the South China Sea，its water vapor channels accounted for 53% of all the tracks，and the contribution rate of water vapor transport was 57. 98%. In addition，the water vapor transport in the Western Pacific and Northwest Passage also contributed to the rainstorm. This would be conducive to understanding the formation mechanism of the extraordinary rainstorm in Northwest China，and provided certain references for the future forecast of extraordinary rainstorm in Gansu Province.