Because the power conditioning system（PCS） of superconductor magnetic energy storage（SMES） device can control active power and reactive power rapidly and independently in all four quadrants， its operating characteristics may affect the dynamic characteristic of pilot current differential protection in existing transmission line. An electromagnetic transient model including SMES in dual power transmission system is established， and the effect of SMES's action characteristics on the pilot current differential protection in transmission line is analyzed based on SMES damping generator power swing. The simulation results show that SMES will not affect the normal action of pilot current differential protection while it is damping generator power swing effectively.