The ring-shaped DC distribution network based on Modular Multilevel Converter (MMC) has attracted much attention because of its power supply reliability advantage. The stable operation of MMC ring-shaped DC distribution network is closely related to the fast and reliable line protection method. To meet this requirement, a Pearson correlation coefficient protection method based on fault transient current is proposed. First, extract the line mode components of the current at the beginning and end of the line in the ring-shaped DC distribution network, and use the difference of the line mode current to complete the identification of internal and external faults; according to the characteristics of single-pole grounding and double-pole short-circuit faults; Using the numerical difference of the correlation coefficient between the positive and negative transient currents of the line, the single and bipolar faults are judged, and the fault pole selection is completed. Finally, a ring-shaped DC distribution network model is built in PSCAD/EMTDC for verification. It can operate quickly and reliably within 2ms, can protect the full length of the line, and has strong resistance to transition resistance. In the case of 40db noise, it can still identify faults, has a certain anti-interference ability, and does not strictly require synchronous communication.