The invention relates to an antibacterial agent and a preparation method thereof. The method comprises the following steps of a, adding methacrylatedimethylaminoethyl esteralkyl or benzyl halide, maleic anhydride and solvents into a reaction vessel; performing stirring; introducing nitrogen; raising the temperature to 35 to 60 DEG C; adding initiators; performing reaction for 5 to 12h; then, raising the temperature to 50 to 70 DEG C; continuously performing reaction for 1 to 2h; performing cooling, separation and purification to obtain a product A, wherein the mol ratio of the methacrylatedimethylaminoethyl esteralkyl or benzyl halide to maleic anhydride is (1 to 14) to (6 to 19); b, dissolving the product A in the step a and aminopropyl POSS into methylbenzene; performing separation purification after the reaction for 20 to 30h at 80 to 95 DEG C to obtain a product B, wherein the mol ratio of the aminopropyl POSS and the product A is (1.0 to 1.08) to 1; c, putting the product B in the step b into sodium hypochlorite or sodium hypobromite; performing still reaction for 1 to 2h at the room temperature; performing filtration and drying to obtain an antibacterial agent. The antibacterial agent has the advantages of long effect, high efficiency, heat resistance, wide spectrum, no drug resistance and biological safety.