Abstract: In this article we divide the northwest Pacific Ocean in to eight areas, and analyse the relation between GD flood season(April-September)rainfall and the sea areas. And we analyse relations among the eight areas with factor analysis. The main factor affecting GD flood season rainfall is found. Using its Variance of maximum rotation of the loading matrix, we work out the important main factor and the main variate affecting rainfall. In the end, we set up a forecast equation to the flood season with the SST which has been used in forecasting and proved to be superior to the average level of the last ten years' professional forecasting.