Using the monthly mean precipitation and temperature data at 168 stations in Northwest China (NWC) in June~September 1961-2000, the characterics of precipitation anomaly in NWC in recent 40 years are diagnosed by means of EOF and REOF, at same time, the ECEP/NCAR monthly reanalyzed data are employed to analyse the evolution character of the field of vapour flux and net vapour flux in the northwest influence region of East Asian summer monsoon.The results show that: The space distribution of precipitation anomaly in WNC in June~September can be separated into the areas of seven climatic anomaly;the effect of East Asian summer monsoon on the precipitation in NWC mainly located the regions to the east of 100°E;vapour socrce in influence regions of NWC come from southerly vapour flux;the vapour flux in influence regions of NWC of East Asian summer monsoon is reached to be significant increasing in the strong summer monsoon year, the precipitation in this region is abundant, it is opposite in weak summer monsoon year.