With intensive surface rain gauge data, conventional sounding data, satellite-sensed temperature of black body (TBB) and 6-hourly 1°×1° NCEP reanalysis data, synoptic diagnosis method was used to investigate the rainfall characteristics, weather background, mesoscale systems of convective rainstorms and its formation causes. The results indicated that the rainstorm was triggered by a long-lived moving mesocale convective system. The moisture accumulation was closely related to the long -range moisture transported from the west Pacific Ocean to Zhouqu and its north area by south flow sometimes and east flow sometimes in the outside of tropical depression “Dianmu”. The combined action of a north-west moving easterly wave perturbation on the north side of tropical depression and a south -east moving westerly short wave on the north side of the subtropical high led to the breaking of the subtropical high, which controlled the most part of north China in early days, and provided necessary dynamic condition for the rainstorm formation. The convergence in the south of Gansu province formed by the bypassing flow of north cold air due to the Plateau blocking and the north-moving warm and moist air, the dynamic lifting and thermodynamic effect of the Plateau east slope under easterly flow may also play an important role in the formation of Zhouqu heavy rainfall.