Abstract: Atmosphere and ocean data in tropical Pacific Ocean from 1980 to 2002 and a numerical model are used to analyze the relationship among the evolution of El Niño, westerly anomaly in equatorial Pacific Ocean and subsurface temperature anomaly in equatorial Pacific Ocean.The results show both the westerly anomaly in western equatorial Pacific Ocean(5°S~5°N,120°~160°E)and in central equatorial Pacific Ocean(5°S~5°N, 160°E~160°W)have the same annual variation as El Niño period, but the former has obvious intraseasonal oscillation of 2~3 months.The subsurface temperature anomaly in the western equatorial Pacific Ocean also has the oscillation period of annual scale and interannual scale.Before El Niño events onset, the subsurface water in western equatorial Pacific Ocean continues to be warmer, and strength of westerly anomaly are enhanced and its frequency increases.When the continual(≥3 months)strong westerly anomaly(zonalwind stress anomaly≥0.1×10^-5N/cm²)appear in the central Pacific Ocean (5°S~5°N, 160°E~160°W) and then propagate eastwardly, and at the same time the positive subsurface temperature anomaly propagate to the eastern equatorial Pacific Ocean by Kelvin wave along the thermocline, the massive surface become anomaly warmer than normal SST, then an El Niño event is formed.Finally, the response of subsurface temperature anomaly in the equatorial Pacific Ocean to the westerly anomaly in 1982/1983 El Niño event is well simulated by a Pacific Ocean model.