A new current differential protection（CDP） principle for HVDC transmission lines is proposed in this paper by analyzing the fault characteristics of DC transmission line. Based on the distributed parameter model， the current at any point of transmission line can be calculated with the voltages and currents measured at local and remote convert stations. When a fault occurs on HVDC line， the sum of the current calculated with each station's data depends on fault current and can reach maximum at the fault point. When no fault occurs or a fault occurs beyond HVDC line， the sum of the current is small（it is near to zero in theory）. According to this feature， CDP can be implemented. Compared with traveling wave protection， CDP can effectively identify inner or external fault and can work in all fault process， and it features high reliability， lower sample frequency and simple algorithm. Compared with the traditional CDP used in HVDC line， it is independent of the distributed capacitance over the long transmission line， therefore it can operate well in the transient process and has high speed. The simulation results show that the method can identify inner or external fault reliably and rapidly.