Based on the problem of disease prevention and control in saline soil subgrades in seasonal frozen regions, this study takes the K222+500 section of the Qinghai-Tibet Railway around Qinghai Lake as the research object. By employing theories of unsaturated soil mechanics, principles of linear elastic continuum mechanics, and a multi-field coupling model, a mathematical theoretical framework considering the porous, multiphase, and multicomponent characteristics of saline soil was established. Numerical simulations were conducted to analyze the evolution patterns of the temperature field, porosity, and frost heave displacement under different subgrade structures and combined measures of thermal insulation layers and slope protection. The research results indicate that reducing the subgrade height within the range of 5 m to 6 m can significantly increase the soil temperature in winter and reduce the peak frost heave displacement. Increasing the subgrade width to above 12.8 m can effectively delay the onset of low temperatures and mitigate shallow salt frost heave diseases. The composite technology of thermal insulation layers (10~20 cm) and slope protection (1.0~1.3 m) can synergistically block heat transfer and improve the thermal insulation effect of the soil beneath the subgrade slope and berm. The research findings provide a theoretical basis and engineering reference for the optimal design and disease prevention of saline soil subgrades in seasonal frozen regions.