The invention provides a direct-current transmission line fault online positioning terminal, method and system. The terminal comprises an open-type magnetic ring, a high-frequency pulse generator, a non-contact sensor and a conversion device, the open-type magnetic ring and the high-frequency pulse generator are both connected to a neutral point of the direct-current power transmission coupling capacitor, and the non-contact sensor and the conversion device are installed on a first-stage tower at an outlet of a converter station. According to the invention, the reliability of fault positioningis greatly improved, the precision is less influenced by the length of the direct-current transmission line and the excessive resistance of the fault, and the positioning precision is improved. According to the invention, the direct-current transmission line is segmented through the plurality of distributed fault online positioning terminals, so that the length of the direct-current transmissionline is indirectly shortened. According to the invention, the fault type is identified, the corresponding fault online positioning is selected based on different fault types, the precision is high, and the online positioning of the high-resistance fault is realized.