ummer precipitation of Gansu province is divided into seven regions by using EOF and REOF method,a representative station is selected in each region. An index of summer drought intensity is designed which indicate not only summer precipitation but also drought duration in summer. The index of summer drought intensity was combined with the summer drought probability. Summer drought probability forecast equations of representative stations were established by analyzing general circulation feature index and surface meteorological essential factors and using the method of stepwise regression analysis and stepwise discriminate analysis. The concept of assembly probability forecast is put forward. Moreover,a method evaluating summer drought probability forecasts is given.