The vortex over northeast Sichuan Basin is one of the main heavy rain-producing systems over San Xia region, especially, the vortex with 700 hPa shear line. The analyses of 500 hPa height fields and 700 hPa wind fields with the vortex from May to September of 1973-1985, show the favorable situations for formation of the vortex are:(1) the ridge line of the subtropical High is in the vicinity of 25°N, and 500 hPa. northwest and southwest currents meet over the northeast of the Sichuan Basin.(2) 700 hPa shear line is also over the Sichuan Basin. After the Q-Vector fields of one case of heavy rain by the vortex were analysed, it is found that a convergent zone of 500 hPa Q-Vector existed over the Sichuan Basin in advance of 12-24 hours in which the vortex was generated, and when Q-Vector convergent areas of middle and lower troposphere were overlapped, a vortex would be formed in the overlap area during the period of 24 hours. It is a good clue for forecasting formation of the vortex.