The fast and reliable identification of DC line faults is one of the key technologies for the development of flexible multi-terminal direct current (MTDC)systems based on modular multilevel converters (MMC). a DC grid fault identification scheme based on single terminal current modulus analysis is proposed by analyzing the transient characteristics of short-circuit faults in the line of flexible MTDC grids. The scheme realizes the rapid positioning of the DC line fault by detecting the polarity and magnitude of the dynamic deviation value of the current-mode fault component, and uses the difference of the current zero-mode fault component after the fault to realize the rapid determination of the fault pole. The four-terminal flexible DC grid model of the modular multilevel converter is built on the PSCAD simulation platform. The simulation example shows that the protection can detect the DC line fault quickly and reliably under different fault locations and transition resistances and accurately identify fault poles.