Based the theory of Barnes filtering, an appropriate bandpass filter is designed, with which the mesoscale data were obtained. The heavy rainstorm in Henan in from 21 to 24 July 2005 resulted from the interaction between Typhoon Haitang and midlatitude systems is analyzed. The results show that the mesoscale disturbance low, mesoscale convergence line, and convergence areas within the largescale cyclonic circulation outside the typhoon are the direct influencing systems of the rainfall storm. The largescale circulation provided abundant vapor sources and a potentially unstable condition for heavy rain, but the mesoscale weather system played an important role in the enhancement of vapour convergence and upward transportation.