The purpose of this paper is to discuss the meaning of "Keqi(pathogenic factors)" in ,Treatise on Exogenous Febrile Diseases, and the guiding effect of its thought on clinic. Starting with the discrimination of syndrome of binding in chest and syndrome of stuffiness, it is known that "syndrome of binding in chest" is easy to occur in yang meridian and "syndrome of stuffiness" in yin meridian, and it is concluded that "emptiness in stomach" is not equal to "stomach qi deficiency" . Furthermore, it is known that the pathogenesis of "Keqi" in the corresponding clauses of ,Treatise on Exogenous Febrile disease, is also different. The Keqi in " Keqi invading the diaphragm" refers to the pathogenic heat sinking inside of the superficial yang, while the Keqi in "ascending counterflow of Keqi" refers to stomach qi deficiency and effulgent heart fire.