A novel power system swing unblock criterion of the three-phase faults for the high voltage line protection relay is presented. Through the voltage and current， the protective relay obtains the line impedance parameter， and the voltage of another terminal of the line can then be calculated. The angle between the phasors of the sampled voltage and the calculated voltage changes periodically when the power system swings， and when there is a three-phase fault occurring on the line， this value of the angle is 180° and does not change， according to such a character， a new three-phase fault unblock criterion is presented. The criterion can determine the unblock delay by the actual swing period， easy to use in the current relay device.