Pole I and pole II of the Gezhouba-Nanqiao±500 kV DC transmission system take place line faults on August 23th， 2006 and August 26th， 2007 respectively， the action mode of system protection is different because of the different characteristics of the two faults. The characteristics of two faults and action modes of system protection are analyzed based on the introduction of the fundamental Gezhouba-Nanqiao DC line protection configuration. The analysis of the two faults demonstrates that the line system protection can act accurately when faults happen. The line travelling wave protection and voltage mutational quantity protection can act quickly when metal short circuit fault happens. The low voltage protection can act accurately when high impedance grounded fault happens with large voltage change. The longitudinal differential protection can act accurately after a long time delay when the change of line voltage is slow with normal communication between stations. The voltage reverse and over-voltage may happen on the fault line when the DC line is with permanent fault locking and abnormal communication between stations.