Aiming at the complexity of rock discing mechanics, and the theory being not mature, the paper carried out three-dimensional numerical simulation for rock core discing based on strain soft model. The strain soft theory is introduced firstly, then numerically simulated how the depth of lug boss and original stress affect the rock core destroy pattern(discing if occurs, the thickness of disk), obtained the characters of rock core discing. Along with the drilling depth increases, the stress concentration scope below lug boss also increases, as well as the pressure stress concentration degree of side corner below of lug boss. Rock core discing can be induced respectively by shear action and tension action, sometimes it is combined by both of them. The destroy pattern of them especially destroy sequence are different. The strain soft numerical simulation could be used in original stress feedback analysis, has significant meaning.