Abstract: Deep mines,tunnels,and other deep engineering rock masses are in a three-dimensional high-stress state,and surrounding rocks affected by mining are often subjected to significant step-loading in the vertical direction. The study on the acoustic emission characteristics of rock under true triaxial step-loading is of great significance to the monitoring and early warning of the deformation and destruction of the surrounding rock in deep ground engineering. To this end,a true triaxial step-loading acoustic emission monitoring experiment of sandstone was carried out,the acoustic emission signal characteristics of sandstone during the true triaxial step-loading process were studied,and the evolution process of rock damage based on the number of acoustic emission window waveforms was revealed. The results show that under true triaxial conditions,with the continuous increase of the maximum principal stress,the failure mode of sandstone gradually changes from tensile failure to shear failure. The number of acoustic emission window waveforms has a good response to the internal stress changes of sandstone and can directly reflect the internal damage evolution of the rock. Therefore,a damage variable and damage evolution model based on the cumulative window waveform number is proposed. The sandstone damage-strain relationship canbe divided into 4 stages. In the initial damage stage,three-way simultaneous loading leads to compact closure of the internal defects of the rock causing slight damage. In the steady development stage of the damage,the damage increases slowly,and the damage-strain relationship is close to linear; the damage develops rapidly. During the rapid development stage of damage,the strain of the rock does not increase with the increase of stress,and the damage increases rapidly. In the stage of failure,the strain keeps increasing,the internal cracks expand and penetrate to form a macroscopic fracture surface until the rock is unstable and damaged.