The heavy rain process causing mud rock flow calamity in the northwestern Sichuan Basin on 18-20, sept. 2001 is analyzed. The results are obtained as follows: (1) Heavy rain causing mud rock flow is connection with early rain day, intensity of heavy rain, geology, landforms. It is suitable that the forecasting area are divided by geology landforms for the heavy rain causing mud rock flow. (2)The weather influence systems at 500hPa are difference in the early and late period of the heavy rain. (3) The heavy rain is presented under the condition of the strong positive vorticity, convergence, vapor flux convergence in middle and lower troposphere and stronger divergence at upper troposphere. (4) Q-vector convergence band has indicative significance for the heavy rain. Specially, Q-vector analysis is more beneficial when the weather influence system is not obvious.