The resistor divider mode is used for the rotor voltage，which is introduced to protective devices to take part in the rotor earth fault protection calculation.Based on the current common modes of earth fault protection，namely，the Ping-Pong type and voltage injection type，a theoretical analysis of the impact on protective measurement is made from two aspects，the grounding resistance and grounding location，with the corresponding solutions put forward.The ALSTOM units of the Three Gorges are taken as an example and the simulation model is built in PSCAD/EMTDC.Simulation results show that the grounding resistance value and location can be accurately calculated using rotor earth fault protection in the resistor divider mode，avoiding high voltage getting switched into the protection screen directly.Meanwhile，by changing different extraction resistors，it is possible to know whether the grounding point located on the leader cable in the excitation circuit or measuring circuit.