The invention relates to a method for preparing a chitosan modified sizing agent used for sizing terylene super fine denier polyester filaments. The method comprises the following steps of: (1) preparing persulfate solution; (2) adding chitosan into the persulfate solution to obtain a mixture; (3) adding glacial acetic acid liquid into the mixture, wherein the reaction system is refluxed in a closed mode and is placed in a water bath; heating the reaction system to the temperature of 90 DEG C; performing reaction by preserving heat; adding the chitosan; when the viscosity value is tested to be between 8 and 16 mPa.s, adding sodium sulfite solution; lowering the temperature to 30 DEG C; testing the pH value; regulating the pH value of reaction system solution to be 5 to 7 by using NaHCO3 solution; and adding a non-ionic surface active agent. The method is simple, has no pollution, and is suitable for industrial production; and the obtained chitosan modified sizing agent has the advantages of high adhesive force, high stability, convenient size mixing, and easy sizing operation.