The invention discloses a method for one-step synthesis of chitosan graft polycaprolactone cationic copolymer, which takes methane sulfonic acid as reaction solvent and catalyst, amino as protective agent and hydroxyl on chitosan as initiating agent under the protection of inert gas, and adopts a one-step method for ring opening polymerization of caprolactone at the normal temperature. The copolymer synthesized by the method can be widely applied to the aspects of biological tissue engineering material and a nano-micelle medicine carrying system. The method has simple preparation process and environment-friendly synthesis technique; furthermore, the prepared material has good biocompatibility, biological degradability and biological absorption performance, thus being used as tissue engineering scaffold material, artificial skin, artificial blood vessel, wound dressing, medicine transmission, wound dressing material, a biological membrane, adhesion prevention material after operation and the like.