Abstract: As a promising energy supply option for extreme scenarios such as polar regions and islands, the performance of the energy conversion system is crucial for micro mobile nuclear power sources. The air Brayton cycle offers advantages such as high safety, compact structure, and easy accessibility of the working fluid, making it one of the ideal choices for the thermal power conversion system of micro nuclear power sources. However, the open‑air Brayton cycle exhibits a relatively high exhaust temperature, containing a large amount of low‑grade waste heat, which limits the system efficiency. To improve the comprehensive energy utilization efficiency, this study constructs an open‑air Brayton‑Organic Rankine cycle (SBC-ORC) combined system based on a 5 MW heat‑pipe micro‑reactor, and analyzes the influence of pressure ratio, regenerator effectiveness, and ORC turbine inlet temperature on the system performance. Meanwhile, a multi‑objective optimization model based on the NSGA-Ⅱ algorithm is established with cycle efficiency, power density, and power-to-mass ratio as the optimization objectives. The optimization results show that the combined cycle using R1233zd(E) as the working fluid exhibits the best thermodynamic performance. After dual‑objective optimization, the cycle efficiency reaches 30.00 % and the power density achieves 125.59 kW/m³; after triple‑objective optimization, the cycle efficiency is 29.19 %, the power density is 137.78 kW/m³, and the power-to-mass ratio is 54.41 kW/t. The research findings can provide theoretical support for the engineering application of micro nuclear power source systems.