The characteristic size of surface roughness on a real vehicle may be comparable to the mean free path of the free stream at high altitudes. At this point, the Navier-Stokes (NS) equations are no longer applicable. Using the Unified Gas Kinetic Scheme (UGKS), which is suitable for all flow regimes, the flow field around a sharp wedge at an altitude of 50 km and a Mach number of 6 is simulated, and the effects of surface roughness on the flow characteristics are analyzed. The results indicate that, compared to a smooth wall, the pressure and heat flux decrease on the left wall of the triangular rough module while increasing on the right wall. Surface roughness significantly enhances momentum exchange between the gas and the wall, resulting in decreases in slip velocity and shear stress on the wall. The total drag increases by 1.33% and 3.61%, while friction is reduced by 9.8% and 9.4%, respectively, when the characteristic size of surface roughness is 1 and 10 times the mean free path of the free stream.