Based on observation data, NCEP/NCAR (1°×1°) 6 h interval FNL reanalysis data、NCEP GFS (0.5°×0.5°) 6 h interval data, Doppler radar and mesoscale ground automatic station data, the environmental condition, triggering mechanism and the evolution of the convection system in the typhoon "Matmo" rainstorm in 2014 were analyzed. The numerical sensitivity experiments for intrusion of cold air intensity were also conducted. The results are as follows. (1) Rainstrom occurred during the process of typhoon transition. The invasion of cold air makes the thermal structure of the typhoon change, atmospheric baroclinic enhance in the periphery of typhoon and the stratification become more unstable, which is beneficial to the development of the mesoscale convective system. (2) The trigger of the convective system is caused by the intrusion of cold air in the boundary layer. The confrontation between cold air and warm air masses causes a strong convergence in the Eastern JiangHuai, which makes the precipitation echo persist. (3) The results of numerical sensitivity experiments show that the intrusion of cold air makes the outer region of the typhoon more statically unstable, which is conducive to the formation of deep vertical motion, prolonging and strengthening mesoscale convective systems. If cold air were too weak, the vertical motion in the convective system would be weak and so would the convection intensity. If cold air were too strong, although strong vertical motion could be developed in the convective system, the life span of the mesoscale convective system would be short, because the short time vertical motion could not be conducive to persistent strong precipitation.