The multi-level nonlinear balanced ω-equation is used to study the Mei-yu from June 29th to July 12th of 1991. The results show that the distribution of the vertical velocity calculated this e-quation can reflect the position of the rain belt properly. The stable condensation heating is the most important factor to the maintenance of the storm rainfall during this period. The horizontal advection of the vorticity is less important than the thermal advection. But the normal advection of the vorticity is important. During this period, the effect of convective condensation heating is very weak.