Based on the moist potential vorticity (MPV) equation containing both heat and mass forcing, the impermeability theorem of the MPV substance (MPVS) is proved. It is clarified that mass forcing induced by intensive precipitation can lead to the MPVS anomaly. The MPVS anomaly is mainly located between 850～500 hPa. With NCEP/NCAR 11 data in Meiyu period in 1999, we find that the MPVS anomaly region corresponds well with the rain belt (include the location and movement of the rainbelt), namely, the MPVS anomaly region is mainly located on the southern side of the Meiyu front, and keeps the same orientation with iso- . According to impermeability theorem of the MPVS, the movement of the MPVS anomaly region on the isobaric can indicate the movement of heavy rain region, which may provide a possible clue to forecast the orientation of the rainbelt.