The invention relates to the technical field of chemistry, and in particular to an organosilicon-modified cationic aqueous polyurethane paper wet strength agent and a preparation method. The organosilicon-modified cationic aqueous polyurethane paper wet strength agent consists of the following materials: polyether diol, tertiary amino polyether diol, polyether triol, dibutyltin dilaurate, isocyanate, cationic hydrophilic chain extender, end-capping agent, glacial acetic acid, acetone, organosilicon modifier and water. The invention introduces tertiary amino groups into both a soft segment structure and a hard segment structure, quaternary ammonium cations are generated after acid neutralization, consequently, the cationic degree is increased, and the organosilicon-modified cationic aqueous polyurethane paper wet strength agent can be better adsorbed on paper fibers with negative charges, and can be quickly and completely retained; by organosilicon modification, crosslinking density is increased, a chemical network structure is formed, and the organosilicon-modified cationic aqueous polyurethane paper wet strength agent which has a high cationic degree and high wet strength can be formed when used on paper; with water as dispersant, the organosilicon-modified cationic aqueous polyurethane paper wet strength agent is nontoxic, pollution-free and highly compatible, and the existence of free formaldehyde and organic halide in conventional wet strength agents are prevented, so the organosilicon-modified cationic aqueous polyurethane paper wet strength agent is environment-friendly and safe.