ith applying parameters calculated by HLAFS model of NMC,the physical conditions with which associated a persistent severe torrential rain process of the lower and middle reaches of Yangtze river during the Mei yu period in 1996 are analyzed and diagnosed. Wind field,vertical circulation,relevant physical variable field construction and its distribution character of Mei yu front strong torrential rain are presented. All results are benificial to real time operational forecast and research work.