A series of medium manganese test steels of non-carbide bainite structure were prepared by isothermal quenching heat treatment. The evolution of the inner- and outer-portion of corrosion products on the steels (1200 MPa class) and the effect of the microstructure of corrosion products on its corrosion resistance were studied via periodic immersion accelerated corrosion test in 3.5% NaCl solution and electrochemical corrosion measurement so that to simulate the corrosion situation encountered in the ocean splash zone. The results show that after being heated to 920 ^oC for 30 min and then quenched quickly in salt bath of 340 ^oC for 2 h, the steel presents an uniform and fine microstructure with clear lath boundaries, while its yield strength, tensile strength and elongation at breaking are 1297 MPa, 1402 MPa, and 29.3%, respectively. With the increase of corrosion time, the inner portion of corrosion products gradually changed from the loose porous γ-FeOOH in the initial stage to the dense α-FeOOH in the later stage of corrosion. The alloying elements beneficial for enhancing corrosion resistant such as Cr and Cu were enriched in the inner portion of the corrosion products, and stable compounds such as FeCr₂O₄ and CuFe₂O₄ were formed. With the increase of pre-corrosion treatment time, the corrosion current density first increases and then decreases, the corrosion potential first shifts negatively and then positively, the charge transfer resistance increases, and the protective effect of the corrosion product film is gradually enhanced. With the increase of the enrichment degree of corrosion resistant elements in the inner portion of corrosion products, the corrosion current density decreases, the charge transfer resistance increases, and the corrosion resistance increases.