Abstract: In the production process of multi-ribbed composite wall, cracks will occur on the contact interface between RC ribbed-frame and AAC blocks under temperature action because of the large difference of material property between concrete and AAC. To discuss the mechanism of the thermal crack of MRCW, the 3D FEM is adopted to simulate temperature field transformation and the analysis of thermal stress is carried out. The relationship between thermal stress and temperature field transformation is presented. The results show that the FE model in this paper accords with engineering practices, and dramatic temperature-decline is the primary reason of increasing abruptly tension stress on surface. The component surface is highly sensitive to any temperature changes. And the temperature gradient of the surface is higher than that of the inner of the component. So, the weakness part is at the component surface. It is suggested that a gap should be set on the contact interface, which would effectively avoid the crack caused by thermal stresses.