During the initial impoundment of mass concrete dam, the reservoir water with low temperature would be critical to the temperature field of dam body, which affects the deformation of dam body and even results in temperature cracks. Therefore, in order to investigate the thermal stress distribution of mass concrete during the initial impoundment, the concrete was assumed as a continuous porous media in this paper. In consideration of the basic theories of solid mechanics, hydraulics, and thermodynamics, the multi-field coupling equations of unsaturated porous media that include momentum conservation, mass conservation, and energy conservation were provided as the function of displacements, pore liquid pressure, pore gas pressure, temperature, and porosity. The finite element analysis program was then developed. A mass roller-compacted concrete block was considered for coupling analyses on the seepage field, temperature field, and stress field. The analysis results show that the temperature reduction and principle thermal stress of the concrete block considering the multi-field coupling process are greater than those without the coupling effect.