The average beginning and ending period of rainy season in Northwest China was analyzed by using the precipitation data of ten
days by ten days from 131 stations in Northwest China and monthly height grid data of 50 x 100at 500 hPa over Northern Hemisphere from
1960 to 2000 and the relationship between precipitation and 500 hPa height field was analyzed by SVD method in rainy season. The re-
sups show that the rainy phase is from May to September and the distribution style of the beginning and ending period of rainy season is
、omplex} but the first ten days in July to the first ten days in September、an represent the rainy season in  main precipitation area.  The
precipitation in rainy season not only has relationship with the same一period 500 hPa(irc;ulation but also with the previous periods 500 hPa
(irc;ulation from March to May and from December to February but the key areas effected by different period(irc;ulation is great different