Abstract：Based on the uniaxial cyclic loading and unloading test results of the Xiangjiaba sandstone，combined with the mesomechanical analysis of the internal microcracks in rock，the characteristics of stress-strain curves，peak strength，fracture damage mechanical characteristics of brittle rock unloading condition are researched. A computational method for rock damage variable is presented based on the stress-strain curve；the acoustic emission test result shows that this method is reasonable. The experimental test results show that，the peak-strength of sandstone under cyclic loading and unloading conditions is less than that of the uniaxial compressive strength，which is influenced by lots of factors. Elastic constants of computation results show that，the Poisson's ratio is gradually increasing during the cyclic loading and unloading process，while the elastic modulus is increased evidently in the first cyclic loading and unloading process，and then decreases slowly. In the process of cyclic loading and unloading for brittle rock，rock damage will increase gradually，and rock damage will rapidly increase when the microcracks propagation runs into the unstable phase；the rock macromechanical characteristics depend on the mesomechanics response of microcracks.
