The invention relates to a method for preparing a cationic grafting agent with multi-reactive groups, which comprises the following steps: adding N,N-dimethyl diaminopropane or N,N-dimethylaminoethanol and hydrochloride solution of N,N-dimethylamino propanol into cyanuric chloride water suspension at the temperature of 0-10 DEG C, leading the molar ratio of cyanuric chloride to the N,N-dimethyl diaminopropane or the N,N-dimethylaminoethanol and the N,N-dimethylamino propanol to be 1: 0.34, using base for adjusting and controlling the pH to be 3-4, increasing the temperature to 35-45 DEG C after full reaction, and further adding the N,N-dimethyl diaminopropane or the N,N-dimethylaminoethanol, leading the molar ratio of the N,N-dimethyl diaminopropane or the N,N-dimethylaminoethanol to the cyanuric chloride to be 0.4:1, using hydrochloric acid for adjusting the pH to be 1-2 after full reaction, increasing the temperature to 70-85 DEG C, adding epichlorohydrin, leading the molar ratio of the epichlorohydrin to the cyanuric chloride to be 0.7:1, and obtaining a monochlorotriazine ammonium chloride derivative. The grafting agent is used for graft modification of fabric, thereby improving the dyeing effect.