With the increasing difficulty of coordination of conventional four-stage definite-time zero-sequence current relays in the domestic Extra-High Voltage (EHV) power grid, inverse-time zero-sequence overcurrent relays are gradually applied as backup protection. However, the settings of inverse-time relays are determined according to engineering experience rather than theoretical analysis. Such action brings many restrictions to its practical application. To this end, this paper first combines the structural characteristics of grounding transformers installed in the EHV power grid substations and analyzes the natural distribution difference characteristics of zero-sequence current to show the theoretical basis of the simplified setting. Then, the coordination method based on the unified setting principle of inverse-time zero-sequence relays is proposed. Finally, the paper proves that selectivity and sensitivity are still guaranteed after simplification via theoretical analysis. The proposed method is applied to the 500 kV partial transmission network of the domestic certain power grid. The results show that the proposed simplified relay coordination method is effective and reasonable. This work is supported by Science and Technology Project of Northeast Branch of State Grid Corporation of China (No. SGDB0000DKJS1900180).