As one of the reactor types proposed by the GIF, the lead cooled fast reactor has advantages such as high thermal efficiency and excellent natural circulation performance. Coupled with advanced power conversion systems, achieving miniaturization, integration, and modularization is feasible. The natural circulation lead cooled fast reactors eliminate the need for coolant pumps, significantly improving engineering feasibility and the inherent safety features of the reactor. Aiming to meet the new requirements for power supply in remote and mountainous areas, this work proposed a conceptual design of a natural circulation lead cooled fast reactor power system NCLFR-100 with a thermal power of 100 MW. The reactor is a typical pool-type fast reactor with an array of heterogeneous hexagonal fuel assemblies loaded with oxide fuels. This paper provided a brief introduction to the structural parameters and power distribution of the core, completed the thermal–hydraulic design for the reactor. The thermal–hydraulic parameters of the hottest fuel were calculated by a subchannel analysis code. The thermoelectric conversion system of the NCLFR-100 adopts the supercritical carbon dioxide Brayton cycle. To improve heat transfer efficiency, the heat exchanger adopts a microchannel semi-circular cross-section direct current channel structure. Sensitivity analysis was conducted on the channel radius, and an appropriate section diameter was selected to reduce the internal pressure drop of the heat exchanger. The results show that thermal–hydraulic parameters are all below the design constraints. This work can support future reactor research and design, providing a reference for the application of lead cooled fast reactor power system.