Experimental and numerical studies were conducted to investigate the flow over a backward-facing step with finite aspect ratio. Measurements using pressure-sensitive paint, infrared thermal imaging, oil-flow visualization and schlieren imaging were conducted in the Hypersonic and High Enthalpy Wind Tunnel at the University of Tokyo. The results show the complex three-dimensional structure of the flow over a backward-facing step. The numerical simulation was also conducted, and the simulation results reveal that the fluid flow from the open side of the model has significant three-dimensional effect on the flow features. Comparison between CFD and experimental results is also made in this paper. A recirculation bubble found after the step, generated by the interaction of side expansion and recirculating flow, is observed in CFD results. A symmetric local peak pressure and heating region named ‘twin peak’ is also founded by both numerical simulation and experiments.