A vertical type once-through heat recovery steam generator (HRSG) comprising: a low-pressure section comprising a condensate preheater configured to heat feedwater through heat exchange with combustion gases and supply the heated feedwater to a low-pressure drum, a low-pressure evaporator configured to heat feedwater stored in the low-pressure drum and convert the feedwater to steam, and a low-pressure superheater configured to heat the steam separated from the feedwater in the low-pressure drum and supply the heated steam to a low-pressure steam turbine; a medium-pressure section comprising a medium-pressure economizer configured to receive at least a portion of the feedwater heated by the condensate preheater, heat the feedwater through heat exchange with the combustion gases, and supply the heated feedwater to a medium-pressure drum, a medium-pressure evaporator configured to heat the feedwater stored in the medium-pressure drum and convert the heated feedwater to steam, and a medium-pressure superheater configured to heat the steam separated from the feedwater in the medium-pressure drum and supply the heated steam to a medium-pressure steam turbine; and a high-pressure section comprising a high-pressure economizer configured to receive at least a portion of the feedwater heated by the condensate preheater and heat the feedwater through heat exchange with the combustion gases, a multi-stage once-through evaporator configured to heat the feedwater heated by the high-pressure economizer through heat exchange with the combustion gases, and a high-pressure superheater (1340, 1341, 1342) configured to heat the steam heated by the multi-stage once-through evaporator and supply the heated steam to a high-pressure steam turbine; characterized in that: the medium-pressure section further comprises a medium-pressure side desuperheater (1261, 1262) installed at the rear of the medium-pressure superheater to lower the temperature of the steam supplied to the medium-pressure steam turbine; and the high-pressure section further comprises a high-pressure side desuperheater installed at the rear of the high-pressure superheater to lower the temperature of the steam supplied to the high-pressure steam turbine. a low-pressure section comprising a condensate preheater configured to heat feedwater through heat exchange with combustion gases and supply the heated feedwater to a low-pressure drum, a low-pressure evaporator configured to heat feedwater stored in the low-pressure drum and convert the feedwater to steam, and a low-pressure superheater configured to heat the steam separated from the feedwater in the low-pressure drum and supply the heated steam to a low-pressure steam turbine; a medium-pressure section comprising a medium-pressure economizer configured to receive at least a portion of the feedwater heated by the condensate preheater, heat the feedwater through heat exchange with the combustion gases, and supply the heated feedwater to a medium-pressure drum, a medium-pressure evaporator configured to heat the feedwater stored in the medium-pressure drum and convert the heated feedwater to steam, and a medium-pressure superheater configured to heat the steam separated from the feedwater in the medium-pressure drum and supply the heated steam to a medium-pressure steam turbine; and a high-pressure section comprising a high-pressure economizer configured to receive at least a portion of the feedwater heated by the condensate preheater and heat the feedwater through heat exchange with the combustion gases, a multi-stage once-through evaporator configured to heat the feedwater heated by the high-pressure economizer through heat exchange with the combustion gases, and a high-pressure superheater (1340, 1341, 1342) configured to heat the steam heated by the multi-stage once-through evaporator and supply the heated steam to a high-pressure steam turbine; the medium-pressure section further comprises a medium-pressure side desuperheater (1261, 1262) installed at the rear of the medium-pressure superheater to lower the temperature of the steam supplied to the medium-pressure steam turbine; and the high-pressure section further comprises a high-pressure side desuperheater installed at the rear of the high-pressure superheater to lower the temperature of the steam supplied to the high-pressure steam turbine.