Using routine ground observation data, T213 and automatic observation station data, diagnostic analyses are made on the causes and moist potential vorticity for the twice sudden drop temperature in a rare cold wave event from 10 to 11 march 2005 in Hubei Province. The results show that the strong cold air flowing into a warm trough induced the first temperature sudden drop, but the second was resulted in the upper cold advection passing downward and the surface precipitation band moving from west to east. The spatial-temporal evolvement of mpv2 being the horizontal component of moist potential vorticity(MPV) has a good relationship with the development,moving and the temperature sudden drop in the severe cold air.