Abstract: To study the frost heave of rocks in cold regions under temperature gradient with water supply, the frost heave experiments on saturated sandstone under unidirectional freezing conditions are conducted in an open system. The results show that the variation process of the frost heave parallel to freezing direction can be divided into three stages during the freezing process of rocks under unidirectional freezing conditions in an open system, namely, thermal contraction stage, in-situ frost heave stage, and segregation frost heave stage. During the segregation frost heave stage, the frost front tends to be stable, and the frost heave increases continuously in an approximately linear relationship with time. Moreover, the frost heave calculated by water migration amount is close to the measured frost heave parallel to freezing direction during the segregation frost heave stage. The segregation frost heave rate increases with the increase of the average temperature gradient, and the location of the segregation ice is in a linear relationship with the average temperature gradient. Furthermore, a coupled THM frost heave model considering the in-situ frost heave of pore water and the segregation frost heave of migrating water is proposed. In the model, the calculation of the in-situ frost heave is based on the unfrozen water content, and a constraint coefficient is introduced to consider the constraint extent of the rock skeleton to the frost heave of the pore ice. Besides, the calculation of the segregation frost heave is based on the segregation potential theory. Comparisons between the experimental and calculated results show that the proposed THM frost heave model is reliable to calculate the frost heave of rocks under unidirectional freezing conditions in an open system, and to simulate the displacement mutation due to segregation ice layer. Therefore, the proposed THM frost heave model is applicable to the frost heave calculation of rock with frost susceptibility in cold regions.