Abstract: Considering the diversity,complexity and space-power-dependence of electric propulsion(EP),the mass factors for EP system,for propellant and for space power are concerned in space mission planning to utilize its high specific impulse.In this context,the concepts of thruster specific impulse,subsystem specific impulse and mission specific impulse are introduced.Based on their definitions,the relationships among three types of specific impulses are derived.The essential meanings of these concepts are clarified,drawing the conclusion that subsystem specific impulse reflects the efficiency of EP subsystem more accurately compared to thruster specific impulse,while mission specific impulse reveals the overall efficiency of EP for space missions.Their applications in EP space engineering are discussed as well.