The invention provides a polyurethane prepolymer which is novel environment-friendly non-isocyanate polyurethane, does not contain an isocyanate group with high toxicity and high moisture sensitivity, has no pollution to the environment and has no damages to human health. A preparation method provided by the invention has a simple operation process; a technology is optimized and raw materials are readily available; and the reaction selectivity is high and a preparation process is safe and clean. A low-viscosity ultraviolet-cured composition can be obtained through mixing the polyurethane prepolymer provided by the invention with a photoinitiator; a system does not contain any active diluting agent and is not retracted and deformed in a curing process; the low-viscosity ultraviolet-cured composition can be widely used for preparing ultraviolet-cured polyurethane coating, 3D (Three Dimensional) printing photosensitive resin, ultraviolet-cured polyurethane ink or an ultraviolet-cured polyurethane adhesive which has low viscosity, does not contain the active diluting agent, has small irritation to human bodies and is green and environmentally friendly.