In order to provide relevant basic data and theoretical guidance for the operation of hybrid hydrogen gas turbines in the future, this paper studied the effect of methane mixed with hydrogen on the efficiency of 9F class gassteam combined cycle unit. The combined cycle model was established based on Aspen Plus software. The gas turbine model was based on the PG9351FA gas turbine, and the waste heat boiler in the steam turbine model adopted a threepressure reheat structure. The output power and efficiency of gas turbine, steam turbine and combined cycle were analyzed. The results show that with the increase of mass fraction of H2 in the fuel from 0 to 100%, the output power of the gas turbine increases by 5.02%, and the efficiency increases by 1.3%. The output power of the steam turbine increases by 0.59%, but the steam turbine efficiency decreases by 2.9%. Meanwhile, the combined cycle output power increases by 3.43%, and the efficiency increases by 1.2%. Therefore, adding hydrogen into methane can improve the economy of gassteam combined cycle unit.