Abstract: In the shape optimization design of the arch dam， it is necessary to make the concrete in a large scale under compression. When the tensile stress exceeds the ultimate tensile strength of the concrete， local area under tension may shift to compression by the action of the stress redistribution. The concrete having low elastic modulus can be replaced in the region under higher tension to decrease the peak tension stress. By this way， the stress state and the safety factor of the arch dam have been improved. An arch dam with the height of 100 m is taken as an example in the study. The stress states under conditions of temperature rise and temperature drop are analyzed by the direct coupling method of the ANSYS10.0. It is clear that much attention should be paid under the temperature rise condition. After replacement of concrete having the low elastic modulus in the region under higher tension， the peak tension stress decreases to 50%， however， there is no remarkable increase in compression stress at the replacement region. Hence， this method can reduce the stress concentration and improve the safety of the arch dam.