To address the lack of effective setting criteria for frequency-domain protection of flexible DC transmission lines and to mitigate the protection maloperation issue due to lightning interference, this paper proposes a high-reliability non-unit protection based on frequency-domain energy ratio. First, this paper analyzes the frequency-domain expressions of line-mode voltage traveling waves under four typical conditions in flexible DC transmission systems: internal ground faults, external ground faults, line lightning interference, and lightning faults, revealing their frequency-domain distribution characteristics. Based on this, an energy operator is introduced to weight the voltage backward traveling wave amplitude-frequency curve, enhancing the discrimination of high-frequency components. Then, the energy accumulation ratio across different frequency bands is used to establish a criterion for identifying internal and external faults. Additionally, considering the significant differences in energy distribution in the low-frequency range due to lightning interference, an efficient method for lightning interference identification is proposed. Finally, a complete non-unit protection criterion for DC transmission lines is developed, supported by clear theoretical setting principles. Extensive simulation results validate that the proposed protection can quickly and reliably identify line faults while accurately detecting lightning interference, exhibiting strong tolerance to transition resistance and excellent anti-interference capability.