Abstract: A double flue duct horizontal structure was proposed for the supercritical heat recovery steam generator (HRSG), of which the height was determined by comprehensively considering the heat-transfer coefficient, the flue gas resistance and the steel consumption of relevant high-pressure once-through evaporation bundles, and subsequently a thermal calculation was conducted by taking a 397 MW gas turbine as the top cycle in the combined cycle unit. Results show that the optimum flue duct width of HRSG is 11.5 m, and the height of upper and lower flue duct would better be 23 m and 21 m, respectively. When the exhaust temperature of gas turbine is same as that of the HRSG, the output of the gas-steam combined cycle under supercritical steam conditions would be 2.46% higher than that under subcritical steam conditions, with an increase in combined cycle net efficiency by 1.16 percentage points.