Abstract: The rapid assessment on post-earthquake damage levels in reinforced concrete (RC) structures strongly relies on accurately evaluating the residual seismic capacity of individual RC components. To enable an efficient and accurate evaluation, this study derives a theoretical formula for the seismic capacity reduction factor of flexural-dominated RC columns at various drift ratios. The derivation is based on the residual seismic capacity assessment method in Japanese standards, which defines the seismic capacity reduction as the degradation of energy dissipation capacity. The formula proposed is validated against the results of quasi-static tests on four RC columns, demonstrating its applicability. Furthermore, a parametric analysis is conducted to investigate the influence of key design parameters on the residual seismic capacity of RC columns, including axial force ratio, stirrup ratio, axial compressive strength of concrete, and shear-to-span ratio. The findings of this study provide a valuable reference for the rapid assessment of the residual seismic capacity of earthquake-damaged RC columns.