Abstract: For a gas-steam combined cycle unit with long-term shutdown and standby, the flow resistance of flue gas was increased by the tarnishing of rear heating surface finned tube with outer helical fins in the waste heat boiler, which further leaded to the abnormal start-up of the gas turbine. Researches were conducted on the influence of the surface tarnishing status of finned tube on the flow resistance characteristics through on-site data monitoring, numerical simulation, and experiment in a small test bench. Results show that, the flow resistances of No.5 and No.6 models in the waste heat boiler rear flue increase significantly. The main reason of the increase of flow resistance is not the rust accumulation in leeward side of the finned tube, but the increase of rust thickness in both sides. After the rust removal, the flow resistance characteristics can be improved with the thinner fins and the enlarged cross-sectional area, and which are better than those of new finned tubes.