An experimental investigation on the flow separation of a hypersonic laminar boundary layer flow over a compression ramp with angles of 32 deg, 36 deg, and 40 deg is carried out in a Mach 5 hypersonic wind tunnel. The detailed structures are measured by particle image velocimetry (PIV), and some typical flow structures, such as a shear layer, separation shock, recirculation zone and reattachment shock, are clearly captured. In the 32 deg compression ramp flow, the hypersonic laminar flow does not experience flow separation, and the boundary layer always attaches to the ramp surface. When the ramp angle increases to 36 deg, a typical flow separation appears in the hypersonic laminar flow, and a shear layer and reattachment shock arise within the flow field. As the ramp angle increases to 40 deg, the separation shock moves upstream, the reattachment shock moves downstream, and the recirculation zone expands. Proper orthogonal decomposition (POD) analysis is performed on the velocity contours for three cases, revealing the spatial structure of the flow field. As the ramp angle increases, the coherent flow structures are captured more effectively by less POD modes, and there are more coherent structures in the flow field of a large-angle compression ramp. Finally, numerical investigations of the flow separation on three different compression ramps are carried out, and the simulation results are consistent with the measurement results.