Crack is one common damage in key cantilever beam structures and early crack identification is much significant for increasing safety and reducing accidents. However, classical methods based on displacements are less sensitive to small cracks. By introducing the concept of vibration power flow analysis, a vibration power flow model of a cantilever beam with a breathing crack is proposed in this paper. Then the simulations are performed to explore the effects of the depth and location of the crack on instantaneous dissipated and input powers, and sub-/sup-harmonic responses due to small crack are observed. The results demonstrate that the proposed method is much sensitive to small cracks than classical methods and provides a new way of detecting early cracks.