Abstract: A 10 layer moist mesoscale model has been employed to simulate a heavy rain case over Yangtze Valley, China during Mei-Yu season. The rain-band and subsynoptic fields are simulated realistically. The results also show that: ① A θse warm lid existed over heavy rain region, which was maintained and enhanced during rainfall period. It is a moist lid, characterized by the inversion of θse and is different from the dry lid with the severe local storms over middle United States. ② The heavy rain region located at the northern or northeastern part of the lid, where the horizontal gradient of θse is considerable large; besides, the upward motion and the potential instability of the stratified atmosphere over there are significant. ③ The warm moist lid not only restrained the release of potential unstable energy from the under running flow, but also enhanced the potential instability of the layer above. Thus, the lid played a role in providing a favorable thermodynamic environment to the development of deep cumulus convection. ④ Based on the thermal wind vorticity eq and the modeling results, the phenomena of the location of heavy precipitation coincide with the strong ascending area and the maximum potential unstable region over 850—700 hPa can be explained by the coupling of vertical motion and the potential instability, ω▽2σs and σs▽2ω. They would induced the imbalance between the mass field and the thermal field so that the “pumping” secondary circulation would develop over the heavy rain region, and the inertial gravity instability would occur as well. Thus, the lid is also favorable to the development of atmospheric imbalance indirectly. ⑤ Based on the eq of stability and the modeling results, the formation of the moist lid is caused by moisture advection of geostrophic thermal wind and differential geostrophic advection ofθse . Therefore, the existence of moist low level jet in the south or south-west of the rainfall region is favorable to the development of the lid.