To address the significant deviations in predicting the ultimate bearing capacity of prestressed expansion pillars in traditional support design models under high-stress conditions in deep mines, this study, based on the Perry-Robertson buckling theory, used Abaqus finite element software combined with multi-condition numerical simulation analysis to investigate the bearing performance of expansion pillars under different slenderness ratios and eccentric loads. A dual-parameter modified bearing capacity formula was proposed and its applicability and reliability were verified based on field monitoring. Numerical simulation results indicate that prestressed expansion pillars are sensitive to slenderness ratio and eccentricity. Under axial compression, an increase in slenderness ratio reduces the ultimate bearing capacity of prestressed expansion pillars; when the normalized slenderness ratio is less than 0.46, plastic buckling predominates, whereas when it is greater than 0.46, elastic buckling predominates. Under eccentric compression, for short pillars, increasing eccentricity changes the buckling mode of the expansion pillar from "C" shape to "S" shape. The modified bearing capacity formula, through the introduction of piecewise fitting functions for the modified slenderness ratio and bending moment correction factor (R2>97%), reveals the transition mechanism of the pillar instability mode from plastic yielding to elastic buckling. Field monitoring data show that, compared to the standard design values in the "Code for Design of Steel Structures" (GB 50017-2017), the modified bearing capacity formula reduces the relative error with measured field bearing capacity from a maximum of 11.41% to 1.55%, better reflecting the nonlinear bearing capacity attenuation characteristics of prestressed expansion pillars.