In this paper, using the precipitation data of 70 stations in the west of China, we have calculated the precipitation periods by the methods of periodogram analysis and power spectrum. It is found that the prevailing period is the quasi-three year period that appears mainly。to the north of 350°N,i.e. to the north of Qinghai-Xizang Plateau, and it is most apparent on the Plateau's northeast.The puasi-three year period of precipitation appears principally in spring and summer. From 1930s to 1970s, the quasi-three year period of the precipitation is relatively stationary. Its formation may be associated with quasi-three year latitudinal oscillation of the ridge line of southern Asia.In addition, there is a minor period of precipitation in the west of China, i.e. the ten to thirteen year period.