Based on the triaxial test results in the core wall of Maopingxi earth-rock fill dam which is the part of the Three Gorges Project in China，the stress-strain relationship and mechanical properties of asphalt concrete are studied. From the test results，it is found that the stress-strain curve of asphalt concrete included two phases of strain-harding and strain-softing；and that the triaxial test curve was a hump curve，which consists of shear-dilatancy and shear-compression during the shear test. The failure criterion of asphalt concrete was not in good agree with the Mohr-Coulomb criterion. The envelope line of shear strength was nonlinearly changing and downward bending with the increasing of confining pressure. The Nanshui nonlinear model was adopted to describe the characteristics of the stress-strain relationship of the asphalt concrete and compared with the Duncan-Chang nonlinear model. The comparative results indicated that the Nanshui nonlinear model seemed better to describe the mechanical properties of the asphalt concrete.