In view of the problem that the sensitivity of current differential protection decreases when a single-phase grounding fault occurs via transition resistance, a single-phase high-resistance grounding backup protection scheme based on voltage phasor plane is proposed. Firstly, by using the characteristic that the sequence current phase of protection installation point is approximately equal to the one of fault point, the angle between the currents of two ends of the line is calculated. Then, the phase angle difference of single-phase ground protection is obtained by calculating the difference between the measured value and calculated value of the angle between the currents of two ends of the line. Finally, based on the difference between internal and external fault in phase angle difference of single-phase grounding protection, the operating criterion of single-phase high-resistance grounding fault is presented. Simulation results based on PSCAD/EMTDC show that the scheme is immune to the fault phase, transition resistance, fault location, operating condition and other factors. It can accurately reflect internal-fault or external-fault with high sensitivity and reliability, which can be used as an effective supplement of current differential protection. This work is supported by Science and Tecnology Project of State Grid Coporation of China (No. 521104170012).