DC fault detection and DC fault isolation based on change rate of DC voltage in DC grid are studied. The principle of change rate of DC voltage is introduced. The influence factors on change rate of DC voltage are analyzed, including current limited reactor, fault location, fault type, fault resistance and measurement time constant. A four-terminal MMC-MTDC is established in PSCAD/EMTDC and setting values of direct current circuit breakers(DC CBs) are chosen. When change rate of DC voltage exceeds setting values, internal DC faults are judged and DC CBs receive the tripping singal to isolate the DC faults. The feasibility of the proposed protection method is proved by simulating pole-to-ground DC fault and pole-to-pole DC fault.