Polyurethane/polyvinylamine (PU/PVAm) composite colloids were prepared by physically blending synthesized waterborne polyurethane with polyvinylamine. The aim of this research was to investigate the effects of colloid preparation conditions on film-forming properties and their potential applications in cellulose fiber materials. The results reveal that when the mass fraction of polyvinylamine is 60%, the resulting colloidal composite film exhibits an elongation at break of 1112.5%, a tensile breaking strength of 7.39 MPa, and a toughness of 52.7 MJ/m3. Furthermore, the composite colloid was employed as surface sizing agent for paper preparation. It is observed that the handsheets with a sizing amount of 6% show a significant improvement in the tensile index and tear index, with increases of 204% and 131%, respectively, compared to the original paper. This research holds significance for the preparation and application of polyurethane-based composite colloids.