By using conventional observation data, HLAFS grid data and GMS cloud data, physical mechanism affecting weather system and rain-formation, interaction and inter impact between low and middle latitude weather system are analyzed and diagnosed. The results indicate that several strong rain procedures occur in the large-scale background of middle-high latitude in Eurasia. The low-eddy which forms in the east area of Qingzang plateau and develops in the Sichuan Basin and its shear line are the main weather systems. The rain intensification is closely related with interaction between middle and low latitude systems, specific construction of plateau eddys.