Given that polar marine environments differ from temperate oceans by persistently low temperatures and distinctive microbial communities, a systematic understanding of the mechanical damage behavior and stress corrosion cracking (SCC) susceptibility of shipbuilding structural steels under such conditions is urgently needed. In the article a test set with controllable environmental factors was designed and made, was designed and made, which can incorporate temperature (4 ℃ and 18 ℃) and corrosive medium (air, sterile artificial seawater, and bacteria-containing artificial seawater) for simulating the service conditions of polar marine environments. Then after pre-corrosion exposure, the degradation of mechanical properties of EH40 steels was quantified, while their SCC susceptibility was evaluated by slow strain rate tensile testing (SSRT). The results demonstrate that mechanical property degradation of EH40 steel in simulated marine environments is governed by the coupled effects of temperature and microbial activity. After pre-corrosion exposure in all corrosive media, the ultimate tensile strength decreased for EH40 steels, with reductions of 2.9%-7.7%. After being subjected to corrosion in sterile artificial seawater at 4 ℃, the tensile ductility of EH40 steel was better maintained, but the elongation at break in the bacteria-containing artificial seawater was ~1.0% lower than that in the sterile artificial seawater. In contrast, at 18 ℃, pronounced ductility loss occurred in the sterile artificial seawater, whereas the bacteria-containing group showed a 4.8% recovery in ultimate tensile strength (an increase of 26.6 MPa), which may be associated with microbially induced interfacial effects (e.g., a biofilm/corrosion-product layer). The results of SSRT show that SCC of EH40 steel also exhibited strong temperature dependence. Because the SSRT duration was short, corrosion-induced cross-section loss of the test steels was limited, and their strength reduction remained below 2.2%. Nevertheless, the SCC susceptibility increased with temperature: at 4 ℃, the elongation-loss indices in sterile and bacteria-containing artificial seawater were 5.9% and 7.6%, and the reduction-in-area loss indices were 6.9% and 11.2%, respectively; at 18 ℃, the elongation-loss indices rose to 15.4% and 13.3%, and the reduction-in-area loss indices increased to 8.9% and 11.8%, respectively. The enhanced SCC susceptibility may be related to temperature-accelerated anodic dissolution, hydrogen diffusion, and microbial metabolic activity. Compared with the static tensile testing after pre-corrosion exposure, the ductility loss by SSRT was more pronounced for the steels, highlighting the dominant role of stress-environment coupling during dynamic loading, where stress concentration and hydrogen accumulation jointly promote crack initiation and propagation. These results provide a meaningful data basis for service-life assessment of polar ship structures.