The surface flow, pressure distributions, and vortex-shedding properties of the circular cylinder flow in the subcritical regime modulated by a self-excited vibrating rod were experimentally studied in a closed-circuit wind tunnel for Reynolds numbers between $2\,{\times}\,10^4$ and $1.8\,{\times}\,10^5$. The principle of galloping was employed to induce self-vibration of an elastic rod. The natural (uncontrolled) boundary layer showed significantly different flow patterns in two subranges of Reynolds number: $Re_D \,{<}\,0.55\,{\times}\,10^5$ and $Re_D \,{>}\, 0.55\,{\times}\,10^5$. In the low-Reynolds-number subrange, the laminar separation mode, which was similar to the result reported by previous investigators, was observed. While in the high-Reynolds-number subrange, the separation bubble mode, which was rarely discussed previously, was found. The term separation bubble mode was used to denote the existence of secondary recirculation bubbles induced by the reattachment of separated boundary layers. The surface flow modulated by the vibrating rod presented complex variations in these two subranges. Depending on the position of the control rod, the separation bubble mode or the turbulent separation mode might appear because of the increase of the turbulence kinetic energy. Modulating the surface flow patterns could significantly influence the surface pressure distributions on the main cylinder and the vortex shedding in the wake. Forcing the boundary layer at the positions upstream of the natural separation point would drastically lower the values of the minimum pressure coefficient and the base pressure coefficient, and therefore increase the lift. The drag coefficient, however, would not be apparently decreased. The frequency of the vortex shedding in the wake could ‘lock’ to the rod vibration frequency if the position of the vibrating rod was properly adjusted.