According to the phase boundary theory, the monomer polymerization interface, that is, the free radical initiating group, is orderly controlled, so that the polymerization is controlled at the nanometer narrowband scale and a nanometer dispersion system is formed, thereby preparing homogeneous chain segments, nanometer particle size (< 80 nm), narrow distribution pure water-borne polyurethane resin (i.e. pure water-borne nanometer narrowband polyurethane resin). Particle size distribution: 47-120 nm; dispersion index: 0.053-0.87; solid content: 42.7-47.5%; viscosity: 8.72-12.65 mPa·s. Under the premise of the same physical properties, the use of polyurethane resin can be effectively reduced to 25%; the direct nano-polymerization process saves 0.234 kilowatt-hours of electricity compared with the prepolymer process; every 1 ton of water-based polyurethane dispersion produced can replace solvent-based polyurethane can reduce the evaporation of 0.7 tons of organic solvents into the atmosphere.