Abstract: The variability of equatorial undercurrent in tropical Pacific and its relationship with ENSO cycle were analyzed using velocity data from Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean array and Simple Ocean Data Assimilation dataset.The influence of equatorial undercurrent on different pattern ENSO events(eastern pattern and central pattern) was tentatively discussed.The result of Empirical Orthogonal Function(EOF) analysis for the anomaly of equat orial under current showed that the characteristic vect or of first mode mainly reflected the variability of Eastern Pacific equatorial undercurrent and the second mode mainly reflected the variability of Central Pacific equatorial undercurrent.These two main modes could show the basic variability of equatorial undercurrent in the Pacific Ocean.The time coefficients of the two modes have obvious interannual variability and they have obvious negative correlations with the NINO index.It revealed that the inf luence of EN SO cycle on the equato rialunder current was shown mainly by the variability of Eastern Pacific under current.And the equatorial undercurrent affected the ENSO events by the variability of Central Pacific undercurrent.The variation of under current in the central pacific has important influence on the different pattern ENSO events.In the ENSO cycle,when the equatorial undercurrent sing ularly increased in central Pacific,the anomalous temperature in western Pacific subsurface propagated to the east and the SSTA reached eastern Pacific,so the eastern pattern ENSO events occurred.Contrarily when the equatorial undercurrent decreased in central Pacific,the anomalous seawater gathered in central Pacific making the sea surface temperature anomaly in central Pacific first,so the central pattern ENSO events occurred.