Abstract Effect of H2S partial pressure (PH2S) on the tensile properties of A350LF2 steel with and without pre-immersion was investigated by slow strain rate testing and electrochemical impedance spectroscopy. The results show that relative tensile strength and plasticity loss increased with increasing PH2S in the absence and presence of pre-immersion. Greater tensile property damage occurred in steel without pre-immersion compared to that with pre-immersion. The relative plasticity loss increment of steel without pre-immersion was larger than that with pre-immersion as PH2S was increased from 0.1MPa to 1.6MPa. Fractography exhibited flat brittle fracture on the samples without pre-immersion and mixed ductile-brittle fracture on the pre-immersed samples. Furthermore, the embrittled regions on the fracture surface of pre-immersed specimens increased with PH2S. In addition, no cracks existed in the steel after pre-immersion. The cracks on the fracture surface of steel could be attributed to the combination of tensile stress and high hydrogen concentration induced by the hydrogen diffusion and accumulation during the tensile test.