Abstract: The adhesion of caprolactam on the surface of ammonium sulfate crystal was studied by adding various kinds of surfactant and common ion compounds into the mixed system of caprolactam and ammonium sulfate. Test results show that cationic surfactants (DTAB, DTAC, CTAC) and fluorine cationic surfactant can reduce the amount of caprolactam adhered on ammonium sulfate significantly; the addition of Na2SO4, NH4Cl, CTAB and dodecyl betaine made the adsorption of caprolactam become irregular. In an industrial system containing caprolactam, ammonium sulfate and crystallization mother liquid, adding a little of DTAB into the system could effectively reduce the amount of caprolactam adhered on ammonium sulfate particles.