The frost heave is influenced by the distribution of thermal field of the subgrade of passenger dedicated railway line in cold region. Based on the background of the subgrade in Dalian of Dalian-Harbin Passenger Dedicated line and combined the local climate condition and geological data, the thermal field in subgrade is simulated by finite element software; and then the distribution of thermal field in subgrade in different months in winter and the change of thermal field under different years are analyzed. The results show that the thermal field in winter can be simulated by using numerical method. In main areas of subgrade, negative temperatures appear in winter; and the effects of sunward and shady aspects should be considered in the distribution of thermal field in actual analysis. The negative temperatures of thermal field in subgrade decrease with the increasing of years in the same period; and the global warming has a significant effect on the distribution of thermal field.