Abstract: The composite coating on Q345 steel surface was prepared by spraying. The corrosion resistance of the composite coating was tested by salt spray test,the corrosion morphologies was observed by using the SEM,the corrosion rate was calculated,the penetration principle of corrosive media was analyzed by using the electron probe microanalysis( EPMA). The results show that the corrosion rate of Q345 steel with composite coating reduces to approximately 6% and the corrosion resistance has improved. But the corrosion near the scratch was serious. The microstructure of the corrosion product is flocculent loose,but coating can stick firmly to the matrix after the corrosion. Corrosion occurs when Cl-penetrates the primer coating to the matrix. The intermediate coating and top of the coating could prohibit the permeation corrosive fluid,retarding the corrosion.