This thesis investigates the impacts of superconducting magnetic energy storage (SMES) unit on the power system first-swing stability by the impedance model of the SMES unit and EMTP simulations. The impedance model of the SMES unit is established in this thesis for study purpose. It has been concluded that SMES unit can greatly improve the power system first-swing stability. Based on the theoretical analysis and simulation results, the concept of the "Stability Protection Zone" of the SMES unit is proposed. Future work directions are discussed in the conclusion part. / Master of Science