Abstract: Using the routine observation data and NCEP 1°×1° final Operational Global Analysis data, two heavy rain processes occurring on 18 th September, 2011 and 18 th September, 2013 over Northwest of Sichuan Basin have been Analyzed and compared.Resultsreveal that the two processes are caused by westerly trough without vortex and high-temperature. Heavy rain areas are all located in the upward side of Longmenshan. The terrain has an important influence on heavy rain. The water vapor for Sichuan basin could come from the Bay of Bengal, it also could come from the East China Sea with the influence of typhoon. The low-level jet provided thermodynamic, moisture and the dynamic convergence for the heavy rain on 18 th September, 2011. The strong low-level convergence caused by terrain and the high-level divergence are the key factors for the heavy rain which happened on 18 th September, 2013. The westerly trough blocked by subtropical high and east wind component of low-level should be noticed for the heavy rain with low-temperature over northwest of Sichuan basin.