Affected by global warming, the frequent occurrence of extreme climate in China has posed severe challenges to the long-term stability of seasonally frozen soil subgrades. Taking the Wangfan Expressway as the example, the interaction between solar radiation regulation and frequent atypical rainfall was considered, and a multi-physics coupled model under multiple boundary conditions was developed to investigate the evolution patterns of the stability of the of seasonally frozen soil subgrade within 50 years. For the temperature field, integrated data was embedded into composite temperature boundary conditions incorporating solar radiation and convective heat transfer, enabling high-precision mapping of long-term climate variations. In the moisture field, two scenarios were defined: short-duration extreme precipitation events and rainfall events interacting with temperature during the rainy season. The dynamic moisture responses within the subgrade were obtained by the coupling of rainfall data with finite element rainfall loading. The results demonstrate that freeze-thaw cycle damage dominated by solar radiation, along with moisture field reconstruction induced by atypical rainfall, significantly influences the long-term frost heave and thaw settlement deformation. By the 50th year, the maximum frost heave and thaw settlement reaches 68.36 mm and 49.56 mm, representing increases of 35% and 43.6% respectively compared to the first year.