The differential equation algorithm（DEA）based on the resistive-inductive model of a fault loop circuit can be used to make rapid tripping decisions.However，effected by various factors，such as the transient response characteristics of voltage and current transformers，the distributed capacitance，etc，the result of DEA fluctuates up and down around the actual value frequently after a fault occurring.It may lead to relay operating incorrectly.In order to overcome this problem，a scheme in which Hilbert-Huang transform（HHT）is used to extract high-frequency components of the results of DEA，is presented.Through empirical mode decomposition（EMD），the intrinsic mode functions（IMF）of high frequency harmonics can be separated out，and the residue which has monotonic trend and useful for evaluating the impedance of fault line can be obtained.Simulation results show that the proposed scheme can evaluate the impedance with high accuracy.