Abstract: Elaborated here in the essay is a process which is used to obtain the marine-service low alloy steels with different metallographic microstructure, grain size and segregation degree by means of controlling the phase transition temperature. The change of the salt spray corrosion rate of the low alloy steel with different microstructure in simulated seawater environment is analyzed via relevant experiment. Furthermore the correlation between the seawater corrosion resistance and the metallographic microstructure, the grain size and the microstructure segregation is discussed. It is revealed that the finer the grain structure is, the lower the salt spray corrosion rate will be;the small content of bainite in the steel microstructure produces just no obvious effect on the salt spray corrosion-resistance of the steel;and the negative effect by the banded structure on the salt spray corrosion resistance is interacted by the grain size.