The invention discloses a chitosan hydrogel for an antibacterial dressing and a preparation method thereof. The chitosan hydrogel for the antibacterial dressing comprises, by weight, 2 to 6 parts of chitosan, 5 to 20 parts of a water-soluble monomer, 1 to 5 parts of an antibacterial agent polyhexamethyleneguanidine hydrochloride, 0.015 to 0.1 part of a cross-linking agent, 1 to 5 parts of a condensing agent, 1 to 5 parts of an activator, 0.1 to 0.5 part of an initiator and 109 to 145 parts of dimethyl sulfoxide. The preparation method comprises the following steps: introducing a carboxyl group on chitosan in the manner of free radical polymerization; subjecting the carboxyl group introduced on chitosan and the antibacterial agent polyhexamethyleneguanidine hydrochloride to an amidation reaction by using the condensing agent and the activator; and uniformly mixing chitosan for the antibacterial dressing with the cross-linking agent and pouring an obtained mixture into a die for a reaction so as to obtain the hydrogel with good drug resistance, good antibacterial property, a lasting antibacterial effect, high water absorption and good mechanical properties.