The work aims to study effects of hard-segment content on cavitation performance of TDI-BDO-PPG waterborne polyurethane (waterborne polyurethane series mainly with TDI as hard segment, BDO as chain extender, PPG as soft segment) in seawater. TDI-BDO-PPG waterborne polyurethane with different hard-segment content was prepared, and cavitation simulation test was performed to it by using ultrasonic cavitation tester. Structural characteristic peak of the polyurethane, tensile properties of the specimens, 100% fixed elongation permanent deformation rate as well as morphology of the specimens before and after cavitation were studied by using fourier infrared spectrometer, tensile testing machine, laser confocal microscope, etc. Ordinary carbon steel coated with TDI-BDO-PPG waterborne polyurethane exhibited better cavitation resistance than itself. As the hard-segment content increased from 27% to 50% gradually, vibration peak associated with hydrogen bond obviously increased, elongation of the specimens decreased, tensile strength increased, 100% fixed elongation permanent deformation rate increased and cavitation-resistant time gradually shortened. There was no linear relationship between cavitation resistance time and hard-segment content, but better cavitation resistance was exhibited when the hard-segment content was about 30%. After the cavitation, a wide range of fine uniform cracks appeared on the coating containing 50% hard segment. The hard-segment content has a significant effect on cavitation resistance of TDI-BDO-PPG waterborne polyurethane in seawater. TDI-BDO-PPG waterborne polyurethane exhibits excellent tensile property, elastic property and cavitation resistance when the hard content is low. Among them, the waterborne polyurethane coating containing 30% hard segment exhibits the best cavitation resistance.