Abstract: With the development of flexible DC transmission technology, DC grid has attracted more and more attention. However, the rapid rise of the fault current of the DC grid has a prominent contradiction with the weak overcurrent capability of power electronics. The DC line protection needs to complete the fault recognition in a few milliseconds with selectivity, reliability, and sensitivity, which is extremely challenging. A pilot protection based on digital twins was proposed for flexible DC transmission systems. On the basis of considering the frequency variation of DC line parameters, an accurate DC line digital twin model was established. The redundancy feature of state estimation was used to improve the reliability of protection, and the fault was judged by comparing the difference between the measured value and the estimated value. The simulation shows that the proposed protection principle can quickly and reliably distinguish the internal and external faults. This principle has the advantages of high fault resistance tolerance and strong anti-interference ability.