The distribution capacitor current of a high voltage DC transmission line will cause the current differential protection to fail to distinguish between faults within and outside the zone immediately. In order to avoid the incorrect trip of protection, the time delay and threshold need to be increased. This greatly reduces the action speed and sensitivity of current differential protection. In order not to be influenced by capacitor current of transmission line, this paper considers the coupling characteristics of positive and negative lines on the transmission line distribution parameter model, and puts forward a new mode current differential protection algorithm for a HVDC transmission line. The algorithm uses common mode compensation current as the fault identification basis and differential mode compensation current as the fault pole selection basis. This mode current differential protection can select fault zone and distinguish fault pole during the fault, with high sensitivity and high speed of movement. Finally, the feasibility and effectiveness of this protection method are verified by PSCAD/EMTDC simulation. This work is supported by the National Key Research and Development Program of China (No. 2016YFB0900600).