Abstract: Based on XBT temperature data during 1955-1999 in the tropical Pacific,the regularity of the warm anomalous signals of the sub-surface ocean temperature in the warm pool region that play important role to the El Nino event occurred in the western Pacific has been analyzed and discussed.The formating mechanism of sub-surface ocean temperature anomaly in the warm pool region in the western Pacific has been announced. The analysis indicates that the relationship between the sub-surface anomalous warming in the western Pacific warm pool and the temperature anomalies in the northern equatorial current(10°N)is close.In the earlier stage of the El Nino event,the anomalous warm water located in the equator central and eastern Pacific subducts toward the warm pool region of the western Pacific along thermocline of the northern equatorial current.Then piles up in the warm pool region and expands toward the equator western Pacific thoroughly.The eastward transport of warm water to the equator central and eastern Pacific will float to the sea surface along the sloping thermocline under the forcing of atmosphere.When anomalous warm wat er achieves a certain strength,it finally leads to the eruption of El Nino event.The westward transport of anoma lous temperature in the northern equatorial current is right the important factor to trigger the sub surface tem perature anomalies in the warm pool region in the western Pacific.So it leads to the key interpreting that El Nino event occurs.