The fault characteristic and the states of DC control system during HVDC transmission line inner and external fault are analyzed in detail considering the influence of DC control system. An equivalent circuit of second-order harmonic based on switch function theory is introduced and analyzed. A new backup protection scheme for DC transmission line is proposed by analyzing the feature of converter firing angle and second-order harmonic in failure period. Converter firing angle is used as operating value and second-order harmonic is used as blocking signal for the backup protection. In addition, theoretical calculation method for the protection settings is provided. Compared with DC line differential protection, the proposed protection can operate with higher speed and will be independent of DC current fluctuations. Compared with derivative under-voltage protection, it can operate well during much higher resistance ground fault. Comprehensive simulation using PSCAD/EMTDC validates that the presented protection scheme can identify inner and external fault reliably and rapidly, and has obvious advantages over DC line differential protection and derivative under-voltage protection.