Abstract: In this paper, the distribution of conditional symmetric stability over and around the Mei-Yu front is analysed in real case studies based on the theory of 2-dimensional flow. The effects of this kind of stability on the quasi-geostrophic frontogenetic vertical circulation and its connections with the vertical structure of the front as well as the large scale horizontal circulation patterns are also examined. The results suggest that slantwise convection may occur due to local conditional symmetric instability after the initially convectively unstable layer is stabilized through vertical convections. Under the condition of small conditional symmetric stability in the warm and moist air, the ascending branch of the frontogenetic circulation can be intensified to produce a rainbelt similar to convections. Therefore, conditional symmetric instability and weak stability could maintain the heavy rainfall observed along the Mei-Yu front. Their occurrence is related to the vertical structure of the frontal region and the horizontal circulation in the mid-and-upper troposp-nere, especially the south Asia anticyclone at 200hPa and the west Pacific subtropical high at 500hPa.