The effect of carbon content on the intergranular corrosion performance of Alloy 800H was studied by thermodynamic software simulation calculation combined with double-loop electrochemical potentiodynamic reactivation (DL-EPR), sulfuric acid-copper sulfate test, and microscopic morphology observation (optical microscope, scanning electron microscope). The results show that the main precipitation phases of Alloy 800H are α-Cr phase, M(C,N), M23C6, η phase and σ phase. At 500∼720°C, the higher the carbon content and the more M23C6 phase, the higher the susceptibility to intergranular corrosion. In the solid solution state, the Alloy 800H has low sensitivity to intergranular corrosion, and the carbon content has no obvious effect on its resistance to intergranular corrosion. In the sensitized state, carbides are continuously precipitated at the grain boundaries, and the intergranular corrosion susceptibility of the alloy increases rapidly.