Abstract: In the numerical simulation of a long-term temperature field in subgrade, the upper thermal boundaries are often determined by the boundary-layer theory, in which the temperature at a certain depth, i.e., the thickness of boundary layer, can be used as a temperature increment plus an average temperature of atmosphere. Because of the lack of a model, the boundary-layer thickness and the temperature increment are usually extracted from the monitored data of subgrade temperature in field. The objective of this study is to develop a model for the determination of the boundary-layer thickness and temperature increment. A theoretical derivation is carried out and leads to a solution of the thickness of boundary layer. Then a numerical model is established to simulate the temperature field in pavement and subgrade. The numerical model is verified through the back analysis of a case study. Furthermore, a sensitive study is carried out to study the impact of solar radiation, pavement materials, pavement structure and pavement radiation on the temperature increment, resulting in an empirical equation and a design chart.