Abstract: The fifth typhoon named HaiTang landed in Fujian in 2005, then it moved to northwest. During this movement, there was a mesoscale convective cloud increasing quickly, within the periphery of cloud cinctures of typhoon connected with tropical convergence cinctures. And the MCS caused heavy rainf all in the eastern and northern part of Wenzhou, which made the more dangerous weather than typhoon. In a word, it was important to research the developmental mechanism of MCS, for the new discovery could contribute to forecasting rainstorm caused by the landing typhoon in the future. Numerical simulation for the typhoon during landing was performed by using mesoscale model WRF in the paper. The model successfully simulated the typhoon track, the intensity tendency as well as the location of rainfall regions induced by typhoon. Especially, the development process of a mesoscale convective system in the typhoon circulation after landing had been successfully simulated. Potential vorticity was a physical quantity which could both reflect atmospheric thermodynamic and dynamic properties, the research work to study the mesoscale convective system applying the potential vorticity theory was not much in China. Therefore, the moist potential vorticity analysis associated with the mesoscale convective system was researched in the paper, so as to reveal the moist potential characteristics of mesoscale convective system in typhoon circulation. The results showed that, during the generation stage, convective instability characterized by MPV1 was a favorable condition for forming MCS, while the vorticity which was caused by the slantwise isentropic surface and the vertical shear of the horizontal wind characterized by MPV2 was an inspiring mechanism. The specific process was that, the atmosphere exhibited the feature of strong convective instability at the low level of convection areas and the southeast areas, which included plenty of erratic energy. The slantwise ascended the stream transfer the erratic energy to northwest, where the stratification stability was small. As decreased, the cyclonic vorticity increased. On the other hand, the vorticity caused by the slantwise isentropic surface and the vertical shear of the horizontal wind inspired the release of convective instability and as a result that the MCS formed. During the development stage, the cyclonic vorticity increased quickly with the persistent decreasing of . Meanwhile, the inclination of isentropic line ( )was bigger than the absolute momentum line (M) at middle level, which exhibited the feature of conditional symmetric instability (CSI) , then the conditional symmetric instability was forced to release by the vorticity resulted from the slantwise isentropic surface and the vertical shear of horizontal wind, and the MCS developed. According to above-mentioned analysis, a conceptual model of MCS development in landing typhoon has been established.