Abstract: A new physical variable, equivalent potential vorticity (EPV),which is used to diagnose the typhoon heavy rainfall, is derived according to the Ertel equation of generalized potential vorticity. Then, the budget equation of the EPV is derived, and each term of this equation is explained. When applying the EPV to the diagnostic analysis of a typhoon heavy rain process, it is found that the area of heavy rain occurred in the downstream of the negative center of EPV and near the maximum negative EPV tendency. Further analysis of the budget equation shows that the EPV can describe the thermodynamical and dynamical characteristic of heavy rain of typhoon. So the EPV is valuable for the prediction of tvphoon heavy rain.