Aiming at the problem of solid–liquid separation after the structure enters water, an experimental platform was set up to realize the vertical and oblique entry of the structure into water. The changes in the flow field during the process of the cylinder entering water were observed by a high-speed camera. Combined with the separation angle to quantitatively characterize the geometrical morphology of the solid–liquid interface, the effects of surface roughness, water entry velocity, and angle on the separation characteristics of the cylinder water entry were further studied. The results reveal that when the cylinder enters water vertically, surface roughness has a significant effect on the separation behavior. As the surface roughness of the cylinder decreases, the separation angle increases first, then decreases, and then increases, and there is a negative correlation with the rough surface hysteresis angle. Secondly, the separation angle of the rough surface cylinder decreases with the increase of the water entry velocity, which is independent of the surface roughness. In contrast, the smooth surface cylinder does not separate with the change of velocity. When the cylinder enters water horizontally and obliquely, the variation of the separation angle with surface roughness and velocity is the same as that of vertical entry.