Aiming at improving the corrosion resistance of 45 steel used in maritime works, an Al-Cr composite alloyed layer is deposited on the surface of 45 steel via a composite technique of plasma ion implantation (PII) and double glow plasma surface metallurgy (DGPSM). Element distributions and phase structures of the Al-Cr composite alloyed layer are investigated via X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Atomic force microscope (AFM), Scanning electron microscope (SEM) and Energy dispersive spectroscopy (EDS). Polarization curves and electrochemical impedance spectroscopy (EIS) are used to study the corrosion resistance and the corrosion mechanisms of the Al-Cr composite alloyed layer, the Al implanted layer and 45 steel in 3.5 wt% NaCl solution. The results show that the Al-Cr composite alloyed layer reveals the best corrosion resistance. The formation of Cr2O3 and Al2O3 as passive films on the surface of the Al-Cr composite alloyed layer, and the inner Fe2AlCr and Al8Cr5 phases with excellent corrosion resistance, hindering the corrosion of Cl- ions effectively. The Al implanted layer exhibits moderate corrosion resistance due to the formation of Al2O3 passive film. The substrate of 45 steel shows the worst corrosion resistance.