Retention aid is only used to improve the process of making small fibers and filler retention rate of chemicals, known as retention aid, if both to speed up the paper filter water, known as retention - filter aid. Early use of aluminum chloride, sodium aluminate and other inorganic products, and later developed into the use of cationic starch, cellulose derivatives and other semi-synthetic products, the effect is not very obvious. Although a certain retention effect, but the amount of large. In recent years, most of the widely used cationic polymer polymers, such as polyacrylamide, polyethyleneimine, polyamide, polyurethane and the like. Their long molecular chains can be bridged between fibers such as fibers, fine fibers, and fillers, and are gradually neutralized with the charge on the fiber surface to form flocculation, thereby increasing retention and increasing water filtration. Negative charge of polyacrylamide in the presence of aluminum ions, the occurrence of complex agglomeration, but also improve the retention rate. The use of retention aid can also reduce the scale of the paper machine, improve the running, reduce the discharge of waste water in the small fiber, filler content, which is conducive to environmental protection.