The invention discloses a paper additive and a preparation method. During preparation, the paper additive comprises raw materials in parts by weight as follows: 0.6-6 parts of an antibacterial agent, namely, polyhexamethylene guanidine hydrochloride, 1-4 parts of carboxymethyl chitosan, 0.4-2 parts of a condensing agent, 0.1-0.5 parts of an activating agent and 0.05-0.2 parts of a cross-linking agent. The preparation method comprises steps as follows: the carboxymethyl chitosan is dissolved in water, then the pH value is adjusted to range from 5 to 5.5 with hydrochloric acid, the condensing agent and the activating agent are added to the mixture for activation, the antibacterial agent, namely, the polyhexamethylene guanidine hydrochloride, is added for a reaction, an obtained reaction liquid is subjected to suction-filtration, washing, drying and crushing and then evenly mixed with the cross-linking agent, namely, N,N-dimethyl bisacrylamide, an obtained mixture is dissolved in deionized water, and the paper additive is obtained. The paper additive has the advantages that the paper additive is broad in spectrum, high in efficiency, low in toxicity, free of drug resistance, high in antibacterial endurance and the like in the antibacterial aspect and has the advantages that the paper additive is capable of significantly enhancing the paper strength, non-toxic and harmless to human bodies and the like in the enhancing aspect.