Abstract: Based on factual analysis that potential vorticity anomaly is closely related to meso－scale convective system and important role of moist in genesis and development of meso－ scale weather，equivalent moist potential vorticity(EMPV) equation in non－uniformly saturated atmosphere is defined on the basis of introduction of moist variant equation and then analyze important factor induced by anomaly of EMPV．Focusing on basic feature of plum humidity fronts，reasons of anomaly of EMPV and vorticity increase induced by arrangement of humidity front and flow field has been analyzed kinetically and kinetic effects of plum rain front and its impacts on the genesis and development of meso－scale system have been primarily revealed．Theoretical analysis show that:EMPV takes into account dynamic，thermal and water vapor effect on atmosphere，and reflects mutual relationship among dynamical variance，thermal change and vapor influence．Through diagnostic analysis，we found that positive and negative anomaly of EMPV have some indication signification for rainstorm area forecast and precipitation trend forecast at single station．To verify the effect of rainstorm forecast on these physical parameter，FNL1°×1°grid data by 4 times a day，output data derived from meso－scale numerical model WRF based on background of FNL1°×1°grid data by 4 times a day and rain data from surface observation are selected to diagnose and analyze EMPV anomaly of a process of continuous precipitation and its average precipitation variation in the Jiang－ huai region in Jun．2010．The results show:the EMPV in the low，to some extent，reflects rainstorm area and its intensity tendency．That is，future precipitation might increase(or decrease) when change value of EMPV shows positive(or negative) anomaly，which gives a qualitative intensity forecast of whether precipitation is reinforced or weakened．Therefore，variation of EMPV is able to provide mobile tendency and intensity variation forecast of rainstorm area and timeliness of one to three hours．