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Potassium Rankine cycle is one of the best thermoelectric conversion schemes for high power output of space nuclear power. Thermodynamic analysis is an important theoretical basis for perfecting cycle system performance. In this work, the output power requirements of space missions are summarized to determine the research objective of 100 kW cycle system, and a thermodynamic analysis model is established. Energy analysis and exergy analysis are introduced to measure the effective use and irreversibility of energy respectively. Secondly, the thermodynamic properties of potassium under saturated and superheated conditions are presented. Finally, the effects of five operating parameters: vapor temperature, vapor pressure, exhaust temperature, condenser outlet temperature and split ratio on the thermal performance of the system are investigated. The results show that the increase of vapor temperature, vapor pressure and main loop flow, the decrease of exhaust temperature are effective measures to improve the thermal performance of the system. Meanwhile, the enhancement of boiler heat transfer performance is the key to reducing system energy loss. This work is of great significance to the improvement of thermal performance and the clarification of optimization direction of alkali metal Rankine cycle system. • Constraints on components and system in the potassium Rankine cycle are given. • Exergy model is introduced to evaluate the irreversibility of the cycle system. • The effect of operating parameters on system performance is investigated. • The enhancement of boiler heat transfer is a key to optimize system performance. • The power requirements of space missions can be matched by parameter adjustment. [ABSTRACT FROM AUTHOR]