Abstract: Three El Nino events are analyzed based on filter method(band pass,low pass)by using ECMWF data.It is clearly shown that tropical intraseasonal oscillation is strong prior to El Nino, corresponding to the occurrence of El Nino event,intraseasonal oscillations obviously weaken and the most obvious variation zone is in tthe tropical eastern Pacific .For quasi stationary wave (with period greater than 90 days),corresponding to the occurrence of El Nino event,its kinetic energy increases obviously and the most obvious variation zone is from Atlantic westward to the tropical western Pacific.The quasi stationary wave kinetic energy is greater in middle(25-35°N)and middle high(40-50°N)latitude than in the tropics.The quasi stationary wave kinetic energy in middle latitude show consistent variation with tropical quasi stationary wave energy variation,that is,corresponding to the occurrence of El Nino event,quasi stationary wave kinetic energy increases in middle latitude.In middle high latitude(40-50°N),quasi stationary wave shows consistent variation with the tropics in Asia continent(100-170°E).We also point out the eastward propagation of tropical intraseasonal oscillation plays an important role to tropical westerly anomaly and occurrence of El Nino.