To address the adverse impacts on the grid caused by sharp changes in electrical quantities during DC grid faults, a transient power pulse factor-based fault detection for flexible DC transmission lines is proposed, using the boundary characteristics of a current limiting reactor. First, based on the characteristic of sharp changes in electrical quantities after a fault, a fault detection criterion is proposed that combines Savitzky-Golay filtering with the power change rate at the positive and negative protection measurement points. Second, an equivalent circuit diagram of the flexible DC grid is established for different fault types, and the transient power magnitude characteristics of the two ends of the upper current limiting reactor on the DC line after a fault are derived. The power change amount is extracted using the impulse factor to constitute fault detection criteria for internal and external faults. Finally, the ratio of the pulse factors of the power at the positive and negative protection measurement points is calculated to construct the fault selection criterion. Simulation results show that the proposed fault detection method can accurately identify the fault types, with no misjudgment in different operating conditions, demonstrating a certain adaptability for fault detection in flexible DC grids.