The present invention discloses a differential protection method of a long-distance ultra-high voltage direct-current (UHV DC) transmission line, and provides a method for properly compensating distributed capacitive current. Voltage current travelling waves are calculated according to line parameters and voltage current at two ends of a detected line, current travelling waves at two sides are compensated by using travelling wave propagation delays of a half of the line, and thus a positive travelling wave and a negative travelling wave of a middle point of the line are selected as calculating current of two sides of differential protection. When no faults exist inside the line or the middle point of the line fails, capacitive current can be fully compensated by the above method, and protection malfunction is prevented; and when positions beyond the middle point inside the line fail, the capacitive current can be partially compensated by the above method, and a ratio of acting current and braking current approximately equals to that when the capacitive current is fully compensated. With adoption of the method, the distributed capacitive current can be effectively compensated under various operating conditions, influences of the distributed capacitive current on DC differential protection can be greatly weakened, and the sensitivity and reliability of DC line protection are raised.