Based on the ECMWF data in summers of 1983,1986 and 1991,authors diagnostically analyze the features and physical causes of drought events over North China and flood events over the mid-low reaches of the Changjiang River. The results show that two patterns of the potential verticity field exist. One can be called "the blocking pattern"(1986),the other is "cold vortex pattern "(1983 and 1991). In addition,it is obvious that the features of water vapor transport and apparent heat sources of atmosphere vary during the different drought/flood periods.