Abstract: Due to the weak boundary characteristics of the LCC-MMC（Line commuted converter-modular multilevel converter）hybrid DC transmission line,the LCC side appears to have a weak capacity to withstand transition resistance,resulting in insufficient adaptability of the existing HVDC traveling wave protection principle. Based on the quantitative analysis of the boundary characteristics and fault characteristics of the DC line,in view of the "spike-fall" characteristic of the first traveling wave head of the LCC side line-mode voltage,this paper constructs a novel main protection principle based on the line-mode voltage recovery ratio of the first traveling wave. The ratio of voltage drop value and rebound value within the protection window is calculated to identify the fault zone,and auxiliary criteria for eliminating protection dead zone and identifying lightning disturbance are proposed. Simulation results show that the scheme can realize fault identification within100 μs,and has better robustness to sampling rate,opposite-side reactor value,transition resistance,lightning disturbance,etc.,which shows an obvious performance improvement compared with the existing traveling wave protection principle.