Based on JTWC, NCEP/NCAR reanalysis data and variational assimilation of TRMM, the activity of cold air and its impacts on Bengal Bay storm heavy rainfall in Yunnan in early summer from 2000 to 2010 were diagnosed. It was showed that Bengal Bay storm initiating heavy rainfall over Yunnan in early summer both landed at the southeast of Bangladesh, north centre coast of Burma, among them about 78 percents of heavy rainfall appeared after Bengal Bay storm landing. Except for eastern edge, the subtotal areas of Yunnan were easy to be affected by Bengal Bay storm. While there was cold air acting, the intensity of heavy-hard rain was stronger and range was wider. The evolution of circulation was closely related to storm heavy rainfall areas. Further discussion showed without cold air, southerly enhanced significantly, and the wind speed maximum sloped northward with height, vertical wind shear was small, moist layer was deep, convective instability occurred by storm warm-wet air was the important reason for heavy rainfall over western Yunnan, moreover topographic forcing also played an important role. Otherwise, while cold air acting, there was an intensive belt of θ_se sloping northward at the beginning of heavy rainfall, warm-wet storm clusters tilt upward-slided along southern isentropic surface, whereas there was dry-cold air in the middle-low troposphere at the side of north, they met at tilt isentropic surface, ascending motion areas sloping northward was bias to warm-wet region, during heavy rainfall periods, cold air changed more dry-cold, compared with the beginning, ascending motion was more stiff. In the aspect of moist potential vorticity, there was high MPV1 downloading to precipitation area along tilt isentropic surface at upper layer of the north side of southern Yunnan, the forcing of down-sliding cold air strengthened the upward-sliding of warm-wet air, meantime, negative center of MPV2 downloaded along intensive belt of θ_se from high troposphere and invaded low troposphere at the north side of heavy rainfall areas, warm-wet air met with dry-cold air at middle-low troposphere, then conditional symmetric instability generated, which resulted in heavy rainfall over central Yunnan and the south areas.