For hypersonic boundary-layer flow, the effects of entropy layer on the boundary layer are investigated by theoretical analysis combining with direct numerical simulation over 5° blunt cones. The boundary-layer similarity is first discovered after entropy swallowing. Entropy swallowing is the phenomenon that the entropy layer gets swallowed by the developing boundary layer downstream. Compared with sharp cones, the boundary layer under entropy layer can be divided into two regions—the outer layer and inner layer. The outer layer is strongly affected by the entropy layer, whereas the inner layer near the wall is mainly dominated by viscous effects. It is found that after entropy swallowing, the entropy-layer effects do not fully disappear. The boundary-layer-edge variables of the blunt cone reach sharp-cone values at entropy swallowing point, while the flow inside the boundary layer takes a longer distance to reach sharp-cone solutions entirely. After that, the entropy-layer effects vanish. The blunt-cone boundary-layer similarity then shows up. Linear stability theory results show that, for blunt cones, the lower branches of the neutral curves always overlap with those of sharp cones, which further proves the existence of the inner layer. The neutral curves of blunt and sharp cones entirely overlap at somewhere downstream of entropy swallowing point, and at this location, the first-mode growth-rate curves of blunt and sharp cones completely overlap. The second-mode growth rates, however, are found to overlap somewhere further, at the location where the blunt-cone similarity shows up.