Abstract: Considering the characteristics of a flexible DC transmission system, in which the fault current increases rapidly and features a large amplitude when a line fault occurs, a complete fault identification of the line must be accomplished within a few milliseconds to ensure the safe and stable operation of the system. Rapidly and reliable identifying faulty lines remains challenging in protecting the lines of flexible DC transmission systems. Hence, this study proposes a new transient protection principle based on corrected voltage. First, a voltage-correction algorithm is proposed that simultaneously utilizes the amplitude and rate of change of voltage after a short circuit to identify faults more sensitively. Subsequently, an integral based on a corrected voltage is used to effectively distinguish between internal and forward external faults. Second, a new directional element is proposed to construct a pilot backup-protection criterion, which solves the problems of low sensitivity under high-resistance faults and the reverse fault misoperation of single-terminal protection. Finally, a four-terminal flexible DC system model is constructed based on the PSCAD/EMTDC simulation platform to verify the effectiveness of the scheme via simulation.