The fault transient process of a multi-terminal hybrid HVDC system is affected greatly by DC control response. The impact of DC control characteristics should be taken into consideration in the analysis of line fault features and research on the protection principle. Based on the characteristics of this type of HVDC system and the control response of different converter stations, the variation tendency relationship between fault transient currents after a DC line fault occurs is studied and analyzed. Then the fault identification criterion based on the sliding mean currents to identify the DC characteristics of internal and external line fault is proposed, and a specific protection scheme is presented. The validity of the proposed protection scheme is verified by PSCAD/EMTDC simulation. The results show that the proposed scheme can quickly and correctly identify line faults including on the T-junction DC bus. It can be implemented with superior performance for a fault transition resitance and with noise interference. Synchronous data is not needed. With fault pole selection capability, the proposed scheme qualifies as fast backup protection for multi-terminal hybrid HVDC transmission lines. This paper is supported by the National Natural Science Foundation of China (No. 52077082).