Abstract: Potential Vorticity Equation (PVE) is derived and simplified to get mesoscale Potential Vorticity Disturbance Tendency Equation (PVDTE) in order to investigate the evolution of mesoscale lows associated with torrential rain. The availability and operability of PVDTE to analyze the evolution of mesoscale lows is verified and the different evolution factors are analyzed with the torrential rain case in Guangxi province. The result shows the following. (1) The evolution of the high-value center of mesoscale disturbance potential vorticity can directly indicate the development and evolution of mesoscale lows. (2) The derived PVDTE containing the diabatic heating action term could be used to reveal the effect of latent heat of condensation heating on the evolution of mesoscale lows. (3) The quantitative analysis of the contribution of condensation heating by PVDTE is an effective method to differentiate the mesoscale lows causing precipitation from the mesoscale lows produced forcibly by condensation heating during the precipitation. The equation has a great significance to research the interaction between condensation heating and different types of mesoscale lows.