The invention discloses a synthesis method for in situ polymerization preparation of ZnO/nanocellulose modified waterborne polyurethane, and aims at providing a composite material with excellent mechanical property, antibacterial property and thermal stability. The method is technically characterized by comprising the following steps: ensuring that polyol, ZnO/nanocellulose composite particles, acatalyst, triethylamine, a small molecule chain extender containing hydrophilic groups and isocyanate are subjected to thermal reaction for a period of time, so as to obtain waterborne polyurethane prepolymer; continuously adding a small molecule chain extender, and performing thermal reaction for a period of time, so as to obtain a prepolymer subjected to waterborne polyurethane chain extension;finally, adding water and performing high-speed shearing and emulsification, so as to obtain the ZnO/nanocellulose composite particle modified waterborne polyurethane. The method solves the problem ofpoor compatibility of ZnO/nanocellulose with waterborne polyurethane matrix in the prior art.