By using the data from 156 meteorological stations in Sichuan Province, the GDAS (Global Data Assimilation System) data and the HYSPLIT4 model, the water vapor transportation of the rainstorm occurring in western Sichuan Basin in 7-11 July 2013 are analyzed quantitatively. The results show that the water vapor mainly comes from the level of 950 hPa and 850 hPa, and the paths and the sources are significantly different. Backtracking one day, the bigvalue zones of water vapor sources at 950 hPa and 850 hPa both are found in the Sichuan Basin area. Backtracking three days, that of water vapor source at 950 hPa is seen still in the vicinity of the Sichuan Basin, but at the level of 850 hPa, the source is tracked to the east of the Bay of Bengal. The main water vapor sources at 950 hPa appear in the Arabian Sea and the South China Sea region, but in the east of Somalia Peninsula at 850 hPa by backtracking nine days. In general, there are five moisture transport paths at the level of 950 hPa, including two in the north and another three in south. There are two paths at the level of 850 hPa in the north and south. Four water vapor sources have been found at 950 hPa. Water vapor coming from the region of Arabian Sea to Bay of Bengal has the maximum contribution rate (44.1%), followed by the IndoChina Peninsula to South China Sea region (33.1%). The Balkhash Lake (15.7%) and the Baikal region (7.1%) contribute less relatively. There are four water vapor sources at the level of 850 hPa， of which the water vapor from the Arabian Sea region (89.4%) along the path of Indian summer monsoon is the most important. The contribution rate of the dry and cold air from the Balkhash Lake to Baikal Region is 6.3%, and water vapor contribution rates from the Bay of Bengal (3%) and local area (1.3%) are the least.