This article analyzes the pathological mechanism of sepsis based on the theory of collateral diseases. The main manifestations of sepsis are gas collateral damage and blood collateral stagnation. The two affect each other,which in turn triggers the early immune inflammatory response (collateral heat),microcirculation endothelial dysfunction (collateral phlegm),coagulation disorders (collateral stagnation) and intestinal bacteria /endotoxin translocation and advanced inflammatory mediators (collateral toxin),and eventually leads to the formation of sepsis.