<p>The partial replacement of natural aggregates with bamboo aggregates leads to a reduction in the compressive strength of concrete. Hence, a pivotal research question lies in whether the superior strength and ductility of steel can effectively compensate for this strength loss while concurrently enhancing the cooperative load-bearing behavior of the composite structural system. Bamboo aggregates were incorporated into conventional concrete mixtures, which was then cast into circular steel tubes to fabricate bamboo aggregate concrete-filled steel tube (BACFST) stub columns. An experimental program was conducted on 30 circular BACFST specimens, with the primary variables being the steel tube wall thickness&#xa0;(<i>T</i>) and the bamboo aggregate replacement rate (<i>r</i>). The study quantitatively evaluated the monotonic axial compressive behavior, including failure modes and stress–strain responses. The stub columns were mainly damaged by lumbar bulge flexure, and the stress–strain curves can be categorized into two kinds: medium constraint and strong constraint. Moreover, after analyzing the key mechanical indexes, it can be concluded that when the properties of the steel tube are maintained constant, with the increase of <i>r</i>, peak stress (<i>f</i>_<i>cc</i>), strength index (<i>SI</i>), and initial stiffness (<i>E</i>) decrease to different degrees; however, the ductility index (<i>DI</i>) was positively correlated with <i>r</i>, and the value of <i>DI</i> increases about 80.4% at the maximum. Based on experimental data, this study systematically investigated the influence of parameter <i>r</i> on the compressive strength of bamboo-aggregate concrete and proposes a BACFST bearing capacity model. A comprehensive assessment was conducted comparing the proposed model with existing code-specified models, with validation results demonstrating the enhanced rationality and improved predictive accuracy of the proposed model.</p>