The fault characteristics of the flexible DC distribution network are complex, and the rapid and reliable protection of faults is a key breakthrough technology urgently needed for flexible DC distribution networks. Based on this, a longitudinal protection method for DC distribution networks based on the transient voltage Pearson correlation coefficient of DC line current-limiting reactors is proposed. First, the differential characteristics of transient voltages of current-limiting reactors at both ends of DC lines under different fault types inside and outside the region are analyzed. Secondly, the rate of change of the positive and negative line voltages is used as a criterion for protection initiation. The Pearson correlation coefficient of transient voltages of current-limiting reactors at both ends of DC lines is used to identify faults inside and outside the region, and the positive and negative relationships between the rates of change of positive and negative DC line voltages are utilized to design fault polarity selection criteria. Finally, a simulation model of a flexible DC distribution network is built in Matlab/Simulink to verify the feasibility of the proposed protection method. The results show that the proposed protection method can meet the requirements of rapid action and selectivity for the protection of flexible DC distribution networks, with strong transient resistance capability, minimal impact from communication delays, and a certain degree of anti-interference ability.