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The distribution characteristics and changing trend of the temperature field of the subgrade in the permafrost region were simulated by using an enthalpy value-based numerical method. The Greater Khingan Mountains were selected as a case, and the environmental conditions, including solar radiation, temperature, wind speed, route trend, and subgrade construction mode, were analyzed. Results demonstrated that the values calculated by the established model agreed with the measured values in temperature holes, which indicated the feasibility of the model. The temperature field of the subgrade in the permafrost regions changed in an approximately cyclical fluctuation of the cosine form. The annual changing curve of ground temperature slowed down gradually with the increase of depth, accompanied by a phase lag. A noncohesive state was observed within a certain depth range between the active layer and permafrost in cold winter. The maximum thawed depth increased linearly as time increased. The rate of subgrade melting presented nonlinear changes with increasing time. The growth rate was fastest in the first five years. After 5 years, subgrade melting decreased generally with time and became stable after 15 years, reaching zero at the 30th year. [ABSTRACT FROM AUTHOR]