A passive cooling system of a nuclear power plant includes a steam generator, a cooling water storage tank, a water cooling heat exchanger, an air cooling heat exchanger, a divergence valve, and a cooling tower. The steam generator generates steam by heat exchange with a primary coolant system, and the cooling water storage tank stores cooling water therein. The water cooling heat exchanger is disposed in the cooling water storage tank, and the air cooling heat exchanger is connected to the steam generator. The divergence valve is controllable to divert steam from the steam generator into both the water cooling heat exchanger and the air cooling heat exchanger. Each of the cooling water storage tank, the water cooling heat exchanger, and the air cooling heat exchanger are located in the cooling tower.