The rapid and effective identification of DC line faults is one of the key requirements for the development of a Medium-Voltage DC (MVDC) distribution system based on the Voltage Source Converter (VSC). The DC line fault current is characterized by rapidly increasing speed, and the overload capacity of the power electronic device in the system is low. Therefore, based on an MVDC distribution system with DC line current-limiting reactors, the DC line fault mechanism of the system is analyzed. On this basis, a fast identification method for DC line faults in an MVDC distribution system based on the second derivative of the line current is proposed. This scheme can quickly identify the fault side, the fault type and the fault line polarity. Finally, the effectiveness of the proposed fault detection scheme is verified by Matlab, and compared with the fault detection scheme based on the first derivative of line current. The simulation results indicate that the proposed method can still realize the rapid and accurate detection of DC line faults when the fault resistance, fault distance and load change. Moreover, compared with the DC line fault detection scheme based on first-order derivative of line current, the proposed scheme has better selectivity. This work is supported by National Key Research and Development Program of China (No. 2018YFB0904700) and Science and Technology Project of State Grid Corporation of China (No. SGJS0000YJJS1801250).