Abstract We numerically investigated two-dimensional laminar flow past a circular cylinder near a moving wall for Reynolds numbers ranging from 60–200. The gap-to-diameter ratio (G/D) between the cylinder and wall varied from 0.2–10. We studied the enhancement of the gap-induced asymmetric flow, which leads to the transition of the dominant frequency of the drag coefficient, and its association with vortical development. Harmonic analysis of the drag coefficient was performed to estimate the contributions of ω and 2ω to the periodic fluctuation of the drag coefficient, where ω is the lift force frequency. The first harmonic, which represented the primary characteristics of the pressure coefficient, was significantly more affected by the gap than the second harmonic. We found that the simultaneous combination of the harmonic components acting on the upper and lower sides of the cylinder determined the dominant frequency of the gap-induced asymmetric flow. Typical features of a gap-induced asymmetric flow are shown in the instantaneous trace of the pressure fluctuation. This revealed that the negative pressure fluctuations induced by the gap effect on the lower side were stronger than those on the upper side. This leads to a shift in the position of the zero first harmonic amplitude, ultimately resulting in a phase difference between the two sides. Finally, the dominant frequency transition of the drag coefficient occurred at smaller gaps as the Reynolds number increased.