Abstract: The moist potential vorticity (MPV) equation with both heat and mass forcing is derived. It is clarified that both heat and mass forcing induced by the intensive precipitation in torrential rain systems can lead to moist potential vorticity anomaly. With NCEP/NCAR 1°×1° data, the moist potential vorticity anomaly is diagnosed along the Yangtze River from June 23 to June 26 1999. The results indicate that moist potential vorticity anomaly occurs between 700 hPa and 500 hPa, and the maximum anomaly value is up to 1.4 PVU. Both dynamical and diagnostic methods reveal that moist potential vorticity anomaly regions correspond well to the regions of intensive precipitation.