Abstract: With the development of exploration in deep space, the power demanding for space probe is also increasing. Space nuclear power system based on AMTEC has been paid widely attention due to its excellent characteristics. The mathematical model for exergy analysis of space nuclear power system is presented based on the first and second law of thermodynamics. Futhuremore, according to the specific characteristics of thermal process in the space nuclear power system, the thermal system was divided into six control volumes. The effects of core temperature and electrode current density on the output power and exergy efficiency of total system and six control volume have been studied. And the results can provided guidance for designing and optimizing system performance. The result shows that the increasing of the core temperature shows a positive impact on the output power and exergy efficiency. With the increasing of electrode current density, the output power and exergy efficiency increases first and then decreases, and each core temperature corresponds to an electrode current density where the output power and exergy efficiency are maximum. The largest part of exergy loss is found to be condenser and evaporator through the comparative analysis.