Graft copolymerization of acrylamide onto starch and reaction with mercaptoacetic acid subsequently were studied.The reaction product (St-AM-MCAA) was used in the treatment of wastewater containing low concentration cadmium ion (0.40 mg·L-1) as additive.An investigation was carried out to study the effect of St/AM (mass ratio of starch to acrylamide) on the grafting ratio and grafting efficiency and the content of thiol (SH) group.It was found that the cadmium ion removal percentage by St-AM-MCAA3 reached 89.7% and the residual concentration of cadmium ion in treated water was under 0.10 mg·L-1 in the conditions of pH=8 and additive concentration of 2.0 g·L-1 by selecting a high grafting ratio and thiol content product St-AM-MCAA3 as additive. The ability of St-AM-MCAA3 to remove low concentration wastewater containing cadmium ion was higher than that of St-AM3 by 10%.