The invention relates to a method for protecting a high-voltage DC three-pole power transmission line. The method is used for detecting the ground fault of a DC line and extinguishing fault currents through protection actions. The high-voltage DC three-pole power transmission line comprises a rectifier station, an inverter station, a first pole line, a second pole line and a second pole line, wherein the first pole line, the second pole line and the third pole line are all connected between the rectifier station and the inverter station. The method comprises traveling-wave protection, differential undervoltage protection, line longitudinal differential protection and DC line restarting. The protection of the first pole line, the second pole line and the third pole line is divided into the two steps that (1) the DC voltage variation and the DC voltage variation rate of the first pole line and the second pole line are detected, whether the DC voltage variation and the DC voltage variation rate of the first pole line and the second pole line both meet a setting values or not is judged, and if yes, protecting action is conducted; (2) the DC voltage variation rate of the third pole line is detected, whether the DC voltage rate of the third pole line meets a setting value or not is judged, and if yes, the protection action is protected. Compared with the prior art, the method for protecting the high-voltage DC three-pole power transmission line has the advantage of improving the system safety.