The filtering technique is employed to the grid data sets on 1~5, July, 1991 in the East Asian region. The atmospheric motions are decomposed into the large scale background and the mesoscale perturbation fields. Using the p-coordinate dynamic equation system for moist air, the equations including the nonlinear interactions of the different scale system over the various isobaric surfaces in the two-layer model are obtained. After simplification a physical parameter characterizing the development of the mesoscale system on the lower layer (about 700 hPa) is obtained. The development and attenuation of the system would depend upon the positive and negative of the parameter, i. e., upon the motion, vorticity and temperature fields and their interaction of various scales on the 200, 500 and 700 hPa isobaric surfaces.