In this paper, by employing a three dimensional geomechanics model test whose scale is 1:250, the dam cracking process was observed, and integrity stability was analyzed and evaluated under overloading test. The experimental results show that: the stress and strain of the dam can basically meet the requirements of dam safety under normal water load, and the dam failure mode of the left abutment is different from that in the right side because of different types of dam figure and geological conditions. Firstly, the local slip fracture is formed on the foundation, then extends along the bottom of dam, and leads to instability and final failure. In overloading process, crack initiate from dam heel under twice normal water pressure, and form a small cracking zone, then yielding continuously and crack band appearing in downstream surface under 4.5th normal water pressure, and overall failure under 8.5th normal water pressure. The integrity stability is shown strong enough with experimental dam shape (XLD03 Figure) and the configuration of its foundation.