Abstract: Based on the current measurement data from TOGA-TAO(ADCP) and the assimilation analysis results from SODA(simple ocean data assimilation analysis),this paper analyzes and researches seasonal and annual variations of the Equatorial Undercurrent(EUC) in the equator Pacific,which are related to the warm water eastward propagation along the equatorial Pacific,and reveals that the physics mechanism of the anomalous warm water propagates eastward in the El Nino annual event.The result indicates that the seasonal and annual variation of EUC are very obvious.The vertical depth of EUC is from the sea surface to 300-400 meters.The mean biggest velocity of EUC can reach to 100cm/s along the equator.The longitude change of the biggest center of EUC is basically maintained between 130°W-160°W.The annual change of EUC is very obvious too,and particularly in the period of El Nino event occurs,the EUC appears obvious strengthening phenomenon.Its the biggest zonal velocity components can reach above the 140 cm/s.The EUC in middle-east Pacific has began strengthen before the subsurface warm water eastward propagation from western Pacific warm pool,and its the biggest zonal velocity components center is constantly strengthened along thermocline with eastward propagation,leading the anomalous warm water form the warm pool area in the western Pacific eastward propagation.So,we can think that the relationship between the eastward propagation of the anomalous warm water from the warm pool area in western Pacific and the strength of EUC is very closely,also in other words in El Nino event,the variation of EUC is leading to the important mechanism that the subsurface anomalous warm water of the warm pool area in the western Pacific propagates eastward.