The present invention provides a direct-fired supercritical carbon dioxide power generation system and a power generation method thereof, the system comprising: a combustor for burning hydrocarbon fuel and oxygen; a turbine driven by combustion gas discharged from the combustor; a heat exchanger for cooling combustion gas discharged after driving the turbine, by heat exchange with combustion gas recycled and supplied to the combustor; and an air separation unit for separating air to produce oxygen, wherein a portion of the combustion gas discharged after driving the turbine is branched before being introduced to the heat exchanger and is supplied to the air separation unit.