Based on the monthly precipitation data of four typical stations (Lhasa, Zetang, Xigaze and Gyangze) from 1960 to 2006 in the flood season, the periodic variation of precipitation over 47 years has been analyzed by Mexican hat wavelet analysis. It shows that there are remarkable periodic oscillations of 14-16 a and 2-4 a in the middle reaches of the Yarlung Zangbo River, and stations Xigaze and Gyangze have mainly periodic oscillations of 14-16 a, and stations Lhasa and Zetang with periodic oscillations of 2-4 a. There are eight cycles of droughts and floods in the periodic oscillation of 14-16 a and it shows notable periodic oscillations of 2-4 a in 1964-1976 and 1984-1997. The periodic oscillation of 8 a becomes remarkable after the middle of 1970s. During the past 47 years, the phases of oscillation at different time scales are superimposed in five time periods, leading to extraordinarily heavy precipitation over these periods.