Using conventional observations and data from satellite and Doppler weather radar, etc., two extremely heavy rain events that happened in Sichuan Basin in the early autumn of 2012 are compared and analyzed regarding synoptical conditions, convective activities and forecast verification. The emphasis is put on the MCS activities and ambient conditions which induced the convective torrential rains in warm sector directly. The two rain events shared some features in common such as the strong and stable West Pacific subtropical high, the active plateau trough, and the genesis of southwest vortex in the low level in Sichuan Basin, but due to the different positions of the subtropical high ridge line and the different short wave disturbing intensity of westerlies, remarkable differences of the moving speed of southwest vortex and rainfall locations were induced. The MCS in the first rain event was stable and moved relatively slowly, but during the second rain event a MCC emerged with highly intensive precipitation, moving fast. The analysis result shows that there was a deep moist layer in the first rain process which was very favorable for the heavy precipitation, but a notable dry layer in the mid high level during the second rain event which created much stronger instability, good for the forming of severe convective storms. In a word, the two rainstorm processes have clear convections in warm sector and instability, and the position of Low Level Jet is more important for deciding the initiation of the convection. Finally, the forecast verification indicates that, the numerical model has obvious bias in presenting of convection in warm sector, but by the guidance of detailed mesoscale synoptical conceptual model, some correction can be made by subjective forecasting, improving the predictability of convection in the warm sector.