Abstract: Specific impulse is the core index to measure the efficiency of space propulsion system, and it is an eternal pursuit to continuously improve the specific impulse of space propulsion. At present, complex space mission represented by Jupiter's far-planet interplanetary exploration require space propulsion system to achieve ultra-high specific impulse. This paper systematically summarizes technical routes of ultra-high specific impulse propulsion, and expounds the technical approaches to achieve ultra-high specific impulse from the aspects of electrostatic acceleration mechanism, electromagnetic field acceleration mechanism, new propellant acceleration mechanism, and new energy acceleration mechanism. The key technology of improving specific impulse is analyzed, and the development suggestions of ultra-high specific impulse technology are put forward.