The conventional boundary-dependent protection methods become inapplicable in the mesh flexible DC grid, which equipped with current-limiting reactors at the converter station. At the same time, traditional differential protection methods suffer from long delays and stringent communication synchronization issues. This paper establishes the Peterson model of the mesh flexible DC grid and analyzes the phase-frequency characteristics of its measured surge impedance. It finds that: when forward fault occurs, the high-frequency transient components of line-mode voltage and current measured at the protection are in opposite phase; whereas when a reverse fault occurs, the high-frequency transient components of line-mode voltage and current measured at the protection are in phase. Based on this characteristic, the article proposes a “non-boundary-dependent” pilot protection scheme based on the cosine similarity of the high-frequency components of line-mode voltage and current. Simulation results using PSCAD/EMTDC show that the proposed scheme can quickly and reliably identify internal and external faults with low communication requirements and can withstand high transition resistance.