The work aims to study hot corrosion behavior and mechanism of 45CT and NiCr/Cr3C2 coating samples by preparing the two coatings on P12 steel by adopting high velocity oxygen fuel spray technology. The samples were coated with a film of mixed salts 75%Na2SO4+25%NaCl (mass fraction), then placed in simulated flue gas containing 0.15%SO2 at 650 ℃, and later taken out at certain interval to weigh mass and obtain corrosion kinetics curve. Composition, structure and phase of the sprayed coatings and hot corrosion products were analyzed with XRD and SEM/EDS. The coatings prepared by HVOF featured in compactness and low porosity. The samples experienced mass gain in the corrosion process, but both were subject to mass loss during mass gain. Mass loss of NiCr/Cr3C2 was worse than that 45CT subsequently. Corrosion products of 45CT and NiCr/Cr3C2 were identified as Cr2O3, NiO and NiCr2O4. A dense Cr2O3-rich scale formed on these two kind of coatings sample surface. NiCr/Cr3C2 coating samples generated oxide and sulfide layers on the substrate/coating interface, which might easily led to weak bonding between in the coating and substrate. Corrosion resistance of 45CT coating sample is better than that of NiCr/Cr3C2 coating in the 0.15% SO2 simulated flue gas at 650 ℃.