The application of series compensation equipment in an EHV transmission line decreases the sensitivity of differential protection. A pilot protection based on model parameter identification is proposed to solve this problem. The equivalent impedance of series compensation equipment is taken as the identification parameter. The power frequency impedance equivalent model of series compensation equipment and the distributed parameter model of the line are considered. The identification equations of equivalent impedance are derived by voltages and currents at two terminals. When the external fault occurs, the identification impedance is equal to the equivalent impedance. When the internal fault occurs, the identification impedance is obviously different from the equivalent impedance. The protection criterion is proposed based on the above characteristics. Theoretical analysis and simulation results prove that the proposed protection is easy to set and has high reliability. It is hardly affected by the distributed capacitive current, the protection of series compensation equipment or the system operation mode. Compared with traditional differential protection, it has better sensitivity. It can cooperate with the differential protection to form a high performance pilot protection scheme. This work is supported by the National Natural Science Foundation of China (No. 51877089).