The ballast embankment is one of the positive measures of protecting frozen soils in construction of Qinghai-Tibet railway. Based on data in field, this paper analyzed change of ground temperature under the ballast with two kinds of diameters. In warm season, the action of heat-shield on the ballast with diameters of 5-8 cm is better than that with diameters of 40-50 cm. In cold season, the action of convection is stronger than that with diameters of 5-8 cm. Distribution of ground temperature under the ballast between shade slope and sunshine slope of embankment is asymmetrical, which the ground temperature in sunshine slope is higher than that in shade slope.