In order to research the relation between the cooling effect and ballast layer height, the natural convection index for representing the cooling effect of natural convection of ballast embankment in cold regions was introduced based on the natural convection theory and the thermal conduction theory in porous media. The approximate formula of the natural convection index varying with the height of ballast layer was obtained using a thermal conduction solution of the temperature distribution in the ballast layer. The research shows that the minimum of natural convection index corresponds to the minimum of height of ballast layer in embankment, and the maximum of index corresponds to the maximum of ballast layer height. An approach to evaluating the critical heights of ballast layer based on the approximate formula of the natural convection index was investigated.Examples was presented and numerical check was carried out.The result shows that the critical heights based on the natural convection index can refer to design of embankment in cold regions.