A method of preparing polyurethane prepolymer does not require using a toxic isocyanate monomer (manufactured by harmful phosgene) as a raw material. Epoxy resin and carbon dioxide are used as major raw materials to form cyclic carbonates to be reacted with a functional group oligomer, and then amino groups in a hydrophilic (ether group) or hydrophobic (siloxane group) diamine polymer are used for performing a ring-opening polymerization, and the microwave irradiation is used in the ring-opening polymerization to efficiently synthesize the amino-terminated PU prepolymer, and then an acrylic group at an end is added to manufacture an UV cross-linking PU (UV-PU) oligomer which can be coated onto a fabric surface, and the fabric is dried by UV radiation for a surface treatment to form a washing-resisted long lasting hydrophilic or hydrophobic PU fabric.