In order to solve the problem that traditional fault phase selection methods may fail due to high unbalance or high fault ground resistance in distribution network, this paper proposes a phase selection method of single line to ground (SLG) fault in unbalanced distribution network based on ground fault ratio. First, the mathematical expressions of ground fault ratio amplitude and phase after a SLG fault occurs are established. By deriving the two expressions to the ground conductance and analyzing the monotonicity under different parameters, the variation law of zero sequence voltage amplitude and phase under SLG faults of phase A is finally determined. Then the two change laws are integrated, and a fault phase selection method based on the comprehensive variation law of amplitude and phase is proposed. In addition, the variation law of phase A is extended to phase B and C, and the effects of under compensation, over compensation and full compensation grounding conditions on the proposed method are analyzed. Finally, the variation law of zero sequence voltage and the effectiveness of the proposed fault phase selection method are verified by Matlab simulation. The simulation results show that under the four working conditions with damping rate of 5% and three-phase asymmetry of 4.5%, the traditional fault phase selection method will fail, and the proposed method can ensure the correctness of fault phase selection. This work is supported by the Key Project of Smart Grid Techniques and Equipment of National Key Research and Development Program of China (No. 2016YFB0900604).