The work aims to provide a theoretical basis for practical application to engineering by studying the corrosion behavior and failure mechanism of NiAl composite coating in corrosive environment. NiAl composite coating was prepared on 20# steel substrate by virtue of supersonic arc spraying technology. On this basis, organizational structure of the coating was characterized, and comparative study was performed to explore polarization behavior and EIS behavior of coating and 20 steel substrate in 3.5% NaCl solution. The surface layer of NiAl composite coating was mainly composed of Ni solid solution, NiAl intermetallic compound and few Al diffusing into the surface of the composition through pores. The coating was a typical dense layered structure, of which the porosity of surface layer (Ni) was 5.3% while that of base layer (Al) was 4.4%. The self-corrosion potential of NiAl composite coating was lower than that of the substrate, hence it could play favorable cathodic protection. During early test, corrosive medium pored entered the internal coating through microcracks and pores in the coating since thickness of the composite coating was up to 380 μm. The coating was consumed quickly and few insoluble corrosion products were produced. Coating impedance was lower and corrosion rate was smaller. With the extension of corrosion time, insoluble corrosion products gradually accumulated in the pores and the polarization resistance increased rapidly, playing a role in delaying corrosion. NiAl composite coating exhibits better corrosion resistance after combining the corrosion effect of Ni and the passivation effect of Al. The increase of coating impedance is mainly due to self-sealing effect of the coating.