Using wavelet analysis and singular spectrum analysis（SSA）,precipitation of 26 stations over northwest China during 1951—1999 are analyzed. There is a stronger regional feature of precipitation over western China. Between the Hexi corridor area and the area which includes the Ningxia and the north Shaanxi provinces, there is a reverse trend. Various precipitation in northweastern part can be divided into three precipitation types. According to the precipitation change feature in past 30a over northwest China, the three types are: sharp change (Xinjiang areas); transitional (Hexi corridor area); stable type (it includes the Ningxia and the north Shan'xi provinces). It shows that the precipitation of every area has the different response to the  global warming. The northwest China's precipitation, in common, has a period of 3a and 5~7a, while it exists for 9~14a long period. The periods' significations are changed in spatial-temporal. On the other words, there are many different balance states and it becomes unstable for the precipitation with time. There is inherent period length in every area of the northwest China, while the inherent period occurrs, its various amplitude change better relate to the precipitation trends.