Abstract: The pre-existing fault specimens were made to confirm the actual stratum mechanical properties based on the similarity theory. Then the mechanism of failure mode of pre-existing fault was analyzed under the crack tip stress field theory. The results show that with the increase of load, the microcracks develop gradually and propagate, and the propagation path tends to the loading direction. In the process of crack propagation, multiple branch paths are generated, and then the cracks merge with each other to destroy the specimen. The stress field theory can be used to determine the newly generated fracture direction, which is consistent with the main failure angle of the pre-existing fault.