A series of core-shell type cationic soap-free latex were prepared by using styrene(St), butyl acrylate(BA) and methyl methacrylate(MMA) as main materials and introducing rosin as the functional monomer. In this reaction, starch(CS-8), which has low relative viscosity , was used as the emulsifier and dispersant. The influencing factors of the reaction were especially studied and the optimum conditions were got. Then the products were characterized by using scanning electron microscope (SEM), FT-IR, laser particle sizer, particle charge detector(PCD). The results show that the excellent performance and good sizing effects of SAE are achieved when the reaction conditions are as follows: the starch amount is 8%(wt), the charge for the initiator(APS) is 0.5%, and the amount of rosin is 2%(wt). And the water-resistant performance of paper is improved significantly when the rosin is chosen as the functional monomer.