Based on the T106 objective analysis data obtained from 17 to 21 September of 2001,the physical conditions and possible mechanism of Sichuan ″9.18″ torrential rain are analyzed. The results reveal that northward low level south east jet would cause favorable thermodynamic, moisture and dynamic conditions for the heavy rainfall in the northwestern Sichuan. The strong upward vertical movement, resulted from the convergence ascending of low level south east jet exit region and the local topographic forcing, stimulated strong convective clouds. The heavy rain was amplified to a great extent by local topographic forcing, and the convergence of low level wet air.