A heavy rain event near Gannan and Baicheng of Nenjiang valley during 9-11 August in 1998 is investigated. Diagnostic analysis, mesoscale filter analysis and numerical simulation have been conducted to inquire into the physical mechanism responsible for the above heavy rainfall. It shows that the existence of meso-β scale system is the direct reason of the event. A comparison of physical mechanism is made between Gannan and Baicheng. The results show that terrain elevation and latitudinal secondary circulation contribute to Ganan heavy rain, weak cold air and latitudinal secondary circulation and thermal effect in mid-low troposphere play important role in Baicheng heavy rainfall process. The MM5 model reproduces successfully the development of the two weather processes. Sensitive tests show that latent heat release acts a decisive role in the formation of the meso-β scale systems. Mesoscale terrain affects obviously the location and intensity of precipitation cell, and it makes Gannan precipitation increase. At the same time, it favors the maintenance and development of meso-β scale system. The effect caused by the variation of initial humidity field on simulation of rainfall area and intensity gradually decreases along with the increase of integration period. For Northeast China, the amount of water vapor which is necessary for the formation of precipitation is not only determined by the initial field, but also affected by the water vapor transportation from the tropical area to Northeast China.