<p>This study aims to assess the comprehensive strengthening effect of a steel-ultra high performance concrete (UHPC) composite strengthening method. The axial force-moment interaction curve (<i>N-M</i> curve) was calculated in a novel way, using cross-sectional strains at ultimate states as well as real-time stress measurements for each material. The enclosed area of the <i>N-M</i> curve was defined as a comprehensive performance index for the system. We validate our approach with comparisons to numerical modeling and full-scale four-point bending experiments. Additionally, strengthening effects were compared for different sagging and hogging moments based on material stress responses, and the impact of various strengthening parameters was analyzed. We find that the <i>N-M</i> curve of the strengthened cross-section envelops that of the un-strengthened cross-section. Notably, improvements in flexural capacity are greater under sagging moments during the large eccentric failure stage, and greater under hogging moments during the small eccentric failure stage. This discrepancy is attributed to the strength utilization of strengthening materials. These findings provide a reference for understanding the strengthening effects and parameters of steel-UHPC composite.</p>