Man′s physical constitution and six pathogen belong to yin or yang in nature， therefore when an exogenous disease occurs， ， one or more pathogen competes with the healthy qi of the body， that leads to various pathological changes: 1) evil pathogen transforms according to constitution; 2) constitution transfoms according to evil pathogen only at specific stage; 3) transformation of both constitution and pathogen: 1) different constitution and different pathogen leads to different syndrome; 2) different constitution and same pathogen leads to different syndrome;3) similar constitution and same pathogen leads to similar syndrome.