Abstract: Based on the precipitation data from the surface meteorological stations and the NCEP/NCAR (the National Centers for Environmental Prediction-the National Center for Atmospheric Research Reanalysis) data during 1981-2016, the effects of low-frequency oscillation on the persistent heavy precipitation in Western Sichuan were analyzed using the synthetic analysis and Butterworth filtering methods which provided a theoretical reference for the extended period forecast.The results show that the precipitation in Western Sichuan is mainly characterized by low-frequency oscillation in 15-30 d.During the precipitation period, the low-frequency northeast wave and southwest wave in the low level merge and form a convergence area over Western Sichuan.Before precipitation, a low-frequency anticyclone and a cyclone are generated over the Kara Sea and northwest Asia, respectively.These two systems move southeast to the vicinity of Lake Baikal and the Sea of Japan, respectively, which transport cold air for precipitation in Western Sichuan.The low-frequency cyclone over the Indian Peninsula converts into an anticyclone to transport water vapor from the Arabic Sea to Western Sichuan.On the middle level, the low-frequency northeast airflows from the mid-high latitude propagate southward to the rain area.The low-frequency high pressure and low-frequency low pressure correspond to the low-frequency anticyclone and cyclone in the low level, respectively, which propagates to the southeast and affects the Western Sichuan region.On the upper layer, the propagation of low-frequency waves mainly occur in the mid-high latitude.Before the precipitation, the low-frequency anticyclone and the divergence center formed in the west of Lake Baikal first move to the southeast and then extend to the southwest, which controls the beginning of precipitation over the Western Sichuan area.