Abstract: The rubble mound breakwater is considered as a discrete medium structure. For the first time, the two-dimensional rigid Discrete Element Methods (DEM) is used to investigate the process and mechanism of the rubble mound breakwater stability loss under a wave attack. The calculation model and method can simulate the local stability loss of the rubble mound and the process from a partial stability loss to a total stability loss. The calculation result shows that the larger the wave period and wave height, the bigger the wave attack will be, and accordingly, the bigger damages on the rubble mound breakwater are caused. The partial instability of the rubble mound breakwater is marked by the stability loss of the blocks on both shoulders of the mound. The total stability loss is the result of the accumulative partial instability, the damage of the rubble mound breakwater and the wave theory are verified.