Understanding the failure mechanism of cracked rock mass under the general stress state is essential for underground engineering construction safety. A series of true triaxial fracture tests on the sandstone with single pre-existing flaw were conducted. The failure modes of the cracked sandstone were analysed, and the multi-scale fracture characteristics and mechanisms of the basic types of crack were identified. Moreover, the influences of the stress state and the pre-existing flaw on the rock failure mechanism were summarized. The results indicate that the rock failure mode is controlled by the true triaxial stress and the pre-existing crack. Based on the multi-scale fracture characteristics, the fracture mechanism of the crack. The rise of minimum principal stress σ₃ can significantly reduce the percentage of the shear crack, while the rise of intermediate principal stress σ₂ conduces to the increase of the percentage of the tensile crack. The pre-existing flaw has a certain promoting effect on the initiation of the tensile crack, however, the true triaxial stress is the decisive factor controlling the rock failure mechanism.