The invention provides a supercritical carbon dioxide power generation system and method based on natural gas waste heat recovery. The supercritical carbon dioxide power generation system comprises anair separation device, a combustor, a carbon dioxide turbine, a heat regenerator, a water separator, a first carbon dioxide pressurization device, a carbon dioxide recovery device, a waste-heat-recovery heat exchanger, a Kalina turbine, a Kalina heat regenerator, a first mixer, an ammonia separator, a second mixer, a high-pressure condenser and a gas-liquid separator. According to the supercritical carbon dioxide power generation system, natural gas is taken as fuel, and the full capture of carbon dioxide is achieved by adopting an oxygen-enriched combustion method; in the carbon dioxide turbine, carbon dioxide expands to the subcritical state and achieves a higher expansion ratio than that the supercritical carbon dioxide Brayton cycle, so that the carbon dioxide turbine can do more workand the cycle efficiency can be improved; and during the bottom cycle, waste heat of the supercritical carbon dioxide cycle is recovered by adopting the Kalina cycle, and the concentration of a working medium changes in the working medium phase-change non-isothermal process and cycle process, so that the vaporization process and the heat-source heat release process can be matched well, the irreversible loss in the heat exchange process can be reduced, and the waste heat utilization efficiency can be improved.