Compression and expansion coaxial unit and Brayton cycle system

The utility model provides a compression and expansion coaxial unit and a Brayton cycle system. The compression and expansion coaxial unit comprises a driving shaft, a first expansion impeller, a second expansion impeller, a first compression impeller, a second compression impeller and a radial bearing pair. Wherein the radial bearing pair comprises a first radial bearing and a second radial bearing, and the first radial bearing and the second radial bearing are both located on the driving shaft; the first expansion impeller and the first compression impeller are installed on the driving shaft and located on the side, away from the second radial bearing, of the first radial bearing. The second expansion impeller and the second compression impeller are installed on the driving shaft and located on the side, away from the first radial bearing, of the second radial bearing. According to the compression and expansion coaxial unit, an axial fluid dynamic pressure bearing does not need to be used, the structure is