The construction of prefabricated subway stations in northeastern China necessitates a focused investigation into the behavior of composite cross-section gaskets with ethylene propylene diene monomer (EPDM) and water swelling rubber (WSR) under low-temperature conditions. This research conducted experimental and numerical studies to analyze the mechanical and waterproof properties of WSR-EPDM gaskets treated at low temperatures. The mechanical properties were evaluated through hardness and compression tests at various treatment temperatures. Additionally, the study examined the impact of treatment temperature and loading rate on the load-deformation relationship. Then, finite element (FE) models were developed and validated based on mechanical experimental results of the gasket-in-groove system under various low-temperature conditions. Subsequently, the nonlinear FE model was used to analyze the effects of joint offset and opening angle on assembly load and waterproof performance. The results demonstrated a direct correlation between the treatment temperature and the Shore hardness and assembly load of the WSR-EPDM gasket. Lower treatment temperatures led to higher hardness and assembly load. Notably, at temperatures below −10 °C, the loading rate had a more pronounced effect on the assembly load. Furthermore, the assembly load initially decreased and then increased with an increase in joint offset, while it consistently decreased with a larger opening angle. Additionally, as the treatment temperature decreased, the average contact pressure during operation also decreased. The influences of joint offset and opening angle were negligible when the treatment temperature was below −20 °C. However, the coupling influence of offset or opening angle and treatment temperature on the waterproofness is evident for a treatment temperature between 0 ∼ −15 °C. In summary, this study serves as a useful reference for evaluating the waterproof properties of these gaskets in low-temperature environments.