For saturated quasi-brittle rocks containing randomly distributed microcracks, the interstitial water pressure has an important impact on the internal deformation and strength. Fully considering the microscopic characteristics and evolution of material, a multiscale damage constitutive model with the hydro-mechanical coupling under undrained condition was established to study the damage behaviors of quasi-brittle rocks. This model is based on the linear homogenization method of Mori-Tanaka and the irreversible thermodynamics theory. For the small perturbation, an analytical expression of the strength, suitable for the conventional triaxial test under undrained condition, was derived through the remarkable hydro-mechanical coupling analysis. The proposed multiscale model was finally applied to simulate a series of undrained triaxial compression tests of sandstone, where the laws of stress-strain curves and interstitial water pressure in the mechanical loading process were reasonably demonstrated.