The　impacts　of　cloud-induced　mass　forcing　on　the　development　of　the　moist　potential　vorticity　(MPV)　anomaly　associated　with　torrential　rains　are　investigated　by　using　NCEP/NCAR　1degrees　x　1degrees　data.　The　MPV　tendency　equation　with　the　cloud-induced　mass　forcing　is　derived,　and　applied　to　the　torrential　rain　event　over　the　Changjiang　River-Huaihe　River　Valleys　during　26-30　June　1999.　The　result　shows　that　positive　anomalies　are　located　mainly　between　850　hPa　and　500　hPa,　while　the　maximum　MPV,　maximum　positive　tendency　of　the　MPV,　and　maximum　surface　rainfall　are　nearly　collocated.　The　cloud-induced　mass　forcing　contributes　to　the　positive　tendency　of　the　moist　potential　vorticity　anomaly.　The　results　indicate　that　the　MPV　may　be　used　to　track　the　propagation　of　rain　systems　for　operational　applications.