Abstract: This study first establishes a numerical analysis model for ultra-high-performance concrete (UHPC) columns reinforced with steel bars, and then verifies the validity of this model. Based on this model, parametric analysis is conducted through numerical simulations of 12 000 reinforced UHPC (R-UHPC) columns under eccentric loading, focusing on the influence of slenderness ratios, eccentricity ratios, longitudinal reinforcement arrangements, and UHPC strengths on the second-order effect. The results indicate that the moment magnification factor specified in GB 50010−2010 underestimates the second-order effect of R-UHPC columns. Through sensitivity analysis and regression analysis of the parametric analysis results, a refined calculation formula for the moment magnification factor is derived. Subsequently, an analysis program of the normal section bearing capacity of R-UHPC columns under eccentric loading is developed to determine the contribution of steel fiber bridging in the tensile zone. The analytical results indicate that there is a size-dependent effect in the bridging effect of steel fibers. On this basis, the equivalent rectangular stress reduction coefficient for UHPC in the tensile zone is determined. Finally, a method for calculating the bearing capacity of R-UHPC columns under eccentric loading is proposed, which integrates the modified moment magnification factor with the formula for calculating the bearing capacity of the normal section. A comparison between the bearing capacities calculated using this method and the experimental results from 13 tested R-UHPC columns under eccentric loading collected from existing literature is conducted, showing that the two are in good agreement.