The thickening agent of graft copolymer was synthesized by carboxymethyl starch (CMS) and acrylamide (AM) under the condition of initiator. The optimum synthesis conditions were investigated by orthogonal experiment design. The structure of the product which was obtained under the best reaction conditions was characterized by FT-IR, and the salt sensitivity and temperature stability, shear stability and anti-ageing of the graft copolymer solution in different slurry were measured by rotational viscometer. The results show that the viscosity can reach 280 mPa·s under the reaction conditions as follows: the reaction temperature 60 ℃, the reaction time 7 h, the mass ratio of CMS to AM 1∶6 and the initiator addition amount 0.9%. The CMS-AM graft copolymer in different slurry shows excellent the salt sensitivity and temperature stability, shear stability and anti-ageing, which can satisfy the needs of oil field fracturing fluid for thickening agent.