Based on the comparison of the structure and operation characteristics between the 220 kV two-phase transmission line and three-phase transmission line, and combined with the normal protection configuration of two-phase transmission line at present, this paper summerizes the influence of two-phase transmission line on conventional relay protection device and figures out the disadvantages of the protection configuration of two-phase electric traction power supply line. Considering the advantages of optical line current differential protection and application of differential protection, this paper proposes a new phase-combined line current differential relay for two-phase transmission line. This protection relay contains phase current differential relay and neutral current differential relay. It can satisfy the demand of fast tripping of the 220 kV two-phase transmission line which can greatly improve the stability of the power system and power supply reliability of traction system.