Abstract: A real time X ray CT detecting is conducted to the soft rock deformation and damage process under triaxial condition and a series of images are gained.The crack nucleation of the soft rock occurs far from the strength maximum.The crack propagates from the margin to the internal for a intact sample.The main evolution style is in turn between propagation and enlargement of crack.The yield point is based on the evolution crack on CT scale and the meso process of soft rock before the strength maximum is the evolution of crack on CT scale.