With the development of the DC distribution network, the line protection of the DC distribution network is one of the focuses of DC power grid research. This paper proposes a protection method for radial DC distribution networks based on the current mutation integral. When the fault occurs in the line, the fault current mutation direction of the line is positive, and when the inter-pole fault occurs, the amplitude of the positive and negative pole mutation is not much different, and when the unipolar fault occurs, the change of the fault pole is greater; when the fault occurs outside the line, the fault current mutation direction of the line is negative. According to this characteristic, the protection criterion is constructed by using the current mutation, and the fault criterion is constructed by using the integral amplitude ratio of the positive and negative mutation. The applicability of the method is verified by PSCAD, and it has a strong ability to resist transition resistance and noise.