This in situ corrosion study of steel in sea bottom sediment by the “Plate Transplanting Burying Method” (MD method) showed that the corrosion rate of steel in sea bottom sediment with sulfur reduction bacteria (SRB) can be as high as ten times that in sea bottom sediment without SRB. The experiments in simulating sea bottom sediments with different concentrations of cultured SRB showed that the electrochemical polarization behaviour of steel in sea bottom sediment with and without SRB were different. SRB altered the polarization behaviour of steel significantly by acidifying the environment so that apparent hydrogen depolarization occurred and accelerated the corrosion of steel.