A hybrid power generation system using a supercritical CO 2 cycle, comprising: a boiler; a first working fluid circuit configured to produce electric power by a first working fluid heated in the boiler, wherein the first working fluid is liquid- or gas-phase water and wherein the first working fluid circuit comprises a plurality of turbines (200, 202, 204) driven by the first working fluid discharged from the boiler, wherein at least one of the turbines (200, 202, 204) is configured to be connected to a generator to produce electric power, and a plurality of heat exchangers configured to exchange heat with the first working fluid discharged from at least one of the turbines (200, 202, 204), and the first working fluid is transferred to the boiler via the turbines (200, 202, 204) and the heat exchangers; wherein the heat exchangers are feed water heaters (300, 330) to heat the first working fluid and the first working fluid circuit further comprises a feed water pump configured to supply the first working fluid to a corresponding feed water heater; and a second working fluid circuit configured to produce electric power by a second working fluid, wherein the first and second working fluid circuits share the boiler; wherein the second working fluid is a supercritical CO 2 fluid and wherein the second working fluid circuit forms a closed loop such that the second working fluid is circulated only in the second working fluid circuit; and wherein the second working fluid circuit comprises: - a first heat exchanger configured to recover waste heat of exhaust gas discharged from the boiler, - a second heat exchanger configured to heat outside air introduced into the boiler for combustion of fuel, - a third heat exchanger provided in the boiler to heat the second working fluid, - a second circuit turbine driven by the second working fluid heated through the third heat exchanger, and - a compressor configured to compress the second working fluid that drives the second circuit turbine; characterized in that the second circuit turbine is connected to the feed water pump to drive the feed water pump. a boiler; a first working fluid circuit configured to produce electric power by a first working fluid heated in the boiler, wherein the first working fluid is liquid- or gas-phase water and wherein the first working fluid circuit comprises a plurality of turbines (200, 202, 204) driven by the first working fluid discharged from the boiler, wherein at least one of the turbines (200, 202, 204) is configured to be connected to a generator to produce electric power, and a plurality of heat exchangers configured to exchange heat with the first working fluid discharged from at least one of the turbines (200, 202, 204), and the first working fluid is transferred to the boiler via the turbines (200, 202, 204) and the heat exchangers; wherein the heat exchangers are feed water heaters (300, 330) to heat the first working fluid and the first working fluid circuit further comprises a feed water pump configured to supply the first working fluid to a corresponding feed water heater; and a second working fluid circuit configured to produce electric power by a second working fluid, wherein the first and second working fluid circuits share the boiler; wherein the second working fluid is a supercritical CO 2 fluid and wherein the second working fluid circuit forms a closed loop such that the second working fluid is circulated only in the second working fluid circuit; and wherein the second working fluid circuit comprises: - a first heat exchanger configured to recover waste heat of exhaust gas discharged from the boiler, - a second heat exchanger configured to heat outside air introduced into the boiler for combustion of fuel, - a third heat exchanger provided in the boiler to heat the second working fluid, - a second circuit turbine driven by the second working fluid heated through the third heat exchanger, and - a compressor configured to compress the second working fluid that drives the second circuit turbine; - a first heat exchanger configured to recover waste heat of exhaust gas discharged from the boiler, - a second heat exchanger configured to heat outside air introduced into the boiler for combustion of fuel, - a third heat exchanger provided in the boiler to heat the second working fluid, - a second circuit turbine driven by the second working fluid heated through the third heat exchanger, and - a compressor configured to compress the second working fluid that drives the second circuit turbine; characterized in that the second circuit turbine is connected to the feed water pump to drive the feed water pump.