This paper introduces the line protection action of UHVDC power transmission project from Xiluodu to Zhejiang Jinhua (abbr. Bin-Jin), and points out that the disturbance of DC lines electromagnetic coupling causes non-fault pole voltage derivative protection action. The circuit model for electromagnetic coupling analysis of UHVDC power transmission lines is explored. The analytic equation is deduced and the electromagnetic coupling mechanism and disturbance characteristics are studied in depth. The optimization strategy of voltage derivative protection function based on the RTDS simulation and the analysis of DC voltage waves characteristics in external and internal faults are proposed. The new protection logic verifies its feasibility and reliability through the simulation test.