Modifing wheat straw pulp fibers via in situ graft copolymerization has been regarded as one of the effective approaches to improve paper properties. In this work,modified fibers were prepared by in situ graft copolymerization of caprolactam onto fibers surface using potassium persulphate-sodium bisulfite as a free radical initiator under alkaline medium. The optimum process parameters of the grafting reaction were followed: the ratio of modifier dosage to fiber was 0.5 ,the ratio of initiator dosage to fiber was 0.1, the ratio of sodium bisulfite to potassium persulphate was 4:5, pulp consistency was 5% ,the reaction time was 2h,the reaction temperature was at 60°C. Fourier transform infrared spectroscopy (FT-IR) was performed to confirm the existence of the grafted copolymers on the surface of fibers. Furthermore, the paper physical properties of the grafted pulp are improved remarkably.