Abstract: The intensity index and the ridge index of Western Pacific Subtropical High are defined with NCEP data in this paper. These two indexes are analyzed with wavelet, before each is applied with correlation analysis to Niño 3 index. It is shown that these two indexes all have interannual variability. In addition, there is marked correlation between Niño 3 index and intensity or ridge index on the interannual scale. Based on the above conclusion, a possible mechanism is discussed: On the interannual scale, the ridge line of the WPSH first has anomalous southward moving and its intensity decreases so that positive westerlies anomalies on the south of the WPSH strengthens the equatorial latitudinal westerlies; anomalous westerlies push warm water eastward and result in the anomalous rise and westward propagation of SST in Niño 3 region about two months later and the anomalous rise of SST in the surface-layer in the central part of Pacific after another one month; under the effect of the Hardly cell, the increased SST in the central Pacific is also heating the atmosphere and the WPSH begins to strengthen about 3 months later.