The integration of low-carbon clean renewable energy such as photovoltaics and wind power and diverse loads have projected studies of the DC distribution network into a new direction. To improve the reliability of power supply, the fault characteristics of a distribution network system are analyzed. Considering the early stage of the discharge circuit formed by the DC fault line and the DC side capacitor of the converter, a new pilot protection method suitable for flexible DC distribution lines is proposed. The method is based on the least squares method, and the slope of the curve is fitted by the fault current sampling data to identify the development trend of the current at both ends of the DC line after the fault, and then to identify DC line faults quickly and accurately. Through PSCAD/EMTDC modeling and simulation, the results show that the method is effective and reliable. This work is supported by the National Nature Science Foundation of China (No. 51877185) and the Xinjiang Uygur Autonomous Region University Scientific Research Key Project (No. XJEDU2021I009).