Abstract: The system of pollutant interactions is analyzed and relevant applications are suggested on the basis of experimental schemes ，action sources ，action directions and relationships among actions．There are three patterns of interactions including strengthening，weakening and inducing，seven types of interactions between two factors，and thirteen subtypes of joint effect under four basic types of the joint effect．The philosophy of interactions can promote a good understanding of joint effects of two or more factors on organisms．Then the method of the Half Lethal Dose Index is also corrected and extended．