On the basis of the boundary conditions of different types of fault， this paper analyzes the current fault component of different phases. A novel principle of phase selection based on time-domain correlation analysis is proposed. This principle is not only suitable for the non-periodic component， fundamental component and harmonic component， but also correct in the transient process of the fault. Furthermore， this principle is insensitive to transition resistance and fault location. It can select the fault phase quickly and correctly. In particular， the three-phase fault can be identified correctly. ATP simulation results and physical dynamic data testing results have both verified the effectiveness of this principle.