Nonlinear -equation including four force items and other terms associated with preciPitation arc calculated.The results show that the formation of the heavy rainfall in July July 1994 in Beijing is a joint affect of typhoon and westerly trough.Especially.the centre of heave rainfall is in correspondence with the moisture convergence at the low level,and the maxinum precipitation of 12hours is relevant to the intensification of ascending motion and moisture convergence.The laplace term of temperature advection in the nolinear ω-equation is most contrihutive.