With the rapid development of expressway construction, great attention has been paid to the research of asphalt pavement technology. The water damage issue has always been the focus in global. In the paper, asphalt concrete is taken as three-phase composites consisting of aggregate, mastic asphalt and void based on meso-mechanics. Combine with image treatment technology, the water damage of asphalt concrete is studied through macro-test and meso-test method. And then, the model of meso-mechanics damage of asphalt concrete is presented based on meso-structure images. Numerical simulation of water damage of asphalt concrete is finished.The macroscopically mechanism of water damage is more clearly grasped by the quantitative analysis of the internal structure. According to CT image processing result, internal structure change tendency of the test specimens is analyzed in the process of without water and saturation water fatigue testing of asphalt concrete. The distinguish of the saturation water and without water fatigue testing: The macroscopically crack shape of saturation water is a shear damage, another crack of without water is splitting damage; The difference on the meso-structure lies in: the area of mastic asphalt increases under without water fatigue testing, the area of mastic asphalt obviously reduce under saturation water fatigue testing. The result had reflected the test specimens the mechanism of water damage in the saturation water fatigue testing, namely the asphalt concrete cohesion damage (asphalt film raveling, aggregate exposed area increased gradually). And crack of test specimens evolution、 development have correlate with initial void distribution and major axis orientation of aggregate. At the same time, the damage of asphalt concrete is experimentally studied by ultrasonic technology. The ultrasonic velocity of test specimens is measured before and after fatigue experiment. The relationship between the damage and the descent of ultrasonic velocity is established by damage parameter is defined. According to experimental data, the damage parameter is found to be in