The Brayton cycle of supercritical carbon dioxide (S-CO2), is an ideal choice to replace the outdated power cycle. In order to improve the heat transfer performance, the Print Circuit Heat Exchanger (PCHE) has been attracted more attention, since it has a larger specific heat transfer area, compact structure, high efficiency. Based on this inspiration, the flow and heat transfer mechanism of S-CO2 and liquid metal sodium in a straight horizontal semicircular channel were studied, and the flow and coupled heat transfer was numerically analyzed. The influences of flow direction, Reynolds number and channel diameter on heat transfer performance and pressure drop in the semicircular straight channel were further studied. The results demonstrate that the performance of countercurrent designs 8.5% higher than that of the downstream pattern. The total heat transfer coefficient and pressure drop of both cold and hot channels in PCHE increase linearly with the rise in Reynolds number. The buoyancy effect affects the heat transfer when the pipeline velocity is small. The effect of buoyancy on heat transfer basically disappears when the pipeline velocity is high. When the diameter of the hot side channel is fixed and the diameter of the cold side channel is increased from 0.8mm to 1.1mm, the total heat transfer coefficient in the cold channel is increased by 8%, while the total heat transfer coefficient in the hot channel is increased by 51.6%. Through the above research in this paper, some heat transfer characteristics of sodium and supercritical carbon dioxide in PCHE are obtained, which is contribute to the design and optimization of the heat exchanger in sodium cooled fast reactor.