Abstract: Obser vation facts have shown that during the formation and enhancement of the heavy rain the atmospheric stratification structure is moist neutral, which makes the vertical motion of moist air be in a state without drag and buoyancy. In addition, an increase of rainfall accompanies the formation and active of meso-system. In this paper,the relations between the neutral condition of moist vertical motion and meso-β system are investigated and demonstrated that the neutral condition of moist vertical motion is favorable to the formation and development of meso-β system, and used numerical simulation test to give the confirmation of the previous physical frame.