Cationic imidazole-blocked polyurethane (CBPU) was prepared by the reaction of isophorone diisocyanate (IPDI), Polycaprolactone glycol, methyldiethanolamine (MDEA), ADH, bisphenol A, N-[(1, 1-dimethyl-2-acetyl) ethyl]-β-dihydroxyethylamino propanamide (DDP) and imidazole as blocking agent. Fourier transform infrared spectroscopy (FT-IR), dynamic laser light scattering (DLS) and SEM were used to characterize the structure of CBPU, the particle size and the surface morphology of the paper. Influence of the blocked ratio on the particle size was studied. At the same time, effects of the blocked ratio and the dosage of CBPU on the physical properties of the treated paper were also studied. The experimental results show that structures of tertiary amine group, imidazole ring and urethane group are found in CBPU molecular structure. The particle size is nearly related to the blocked ratio of CBPU. The paper strength, especially the wet strength can be obviously increased with the increasing of the blocked ratio and the dosage of CBPU. The wet strength is 30.32% when the blocked ratio and dosage of CBPU are 10% and 5%.