A summery cold front rainstorm in western mountainous region of Guizhou was analysed with sounding，radar and cloud physical data by airplane observations.The potential instability before front is not large.The lower jet stream has evident daily variation. Vapor supplies and natural ice-crystal seeding from higher altitude above 4500m are important for the forming of rainstorm. A very large cloud-rain transforming rate is present at the top of rainstorm cloud.The consistency of large granules including ice-crystals and liquid-water content amount to maximum in the overcooling part of rainstorm cloud.But for the ordinary rain cloud, the maximum of above mentioned factors are present in the warm part of cloud.It seems that for the rainstorm the total content of large granule and LWC are unimportant, and the developing altitude of cloud are also not so significant as the thickness of overcooling part.Some effects of plateau topography for the rainstorm forming are also discussed.