Abstract： Infectious fever belongs to the category of fire- heat syndrome in traditional Chinese medicine. The pathogenesis of the fire- heat syndrome is mostly due to the internal constraint and obstruction of yang qi，and such fever induced by constraint is called fire constraint syndrome. This paper uses the five dimensions of differentiation (diseases，symptoms，syndromes，people，pathogenesis) in identifying the fire constraint syndrome and summarizes characteristics and laws of its onset， which can help grasp the internal pathogenesis of fire constraint syndrome and contribute to a comprehensive and profound understanding of infectious fever.