This study presents numerical simulations to investigate the impact of aspect ratio on axial compressive strength and failure mechanisms of Recycled Aggregate Concrete-Filled Steel Tubular (RACFST) columns. Recycled aggregate concrete (RAC) is a sustainable, cost-effective, and environmentally beneficial option for the construction sector. The attention given to RACFST columns in recent research underscores their potential in sustainable construction. While most studies have concentrated on circular and square shapes, the exploration of rectangular-shaped sections in the context of RACFST columns requires further attention. A 3D finite element (FE) model is created to investigate the effects of aspect ratio alteration. The model includes both material and geometric nonlinearities. By comparing it to experimental data published in the existing literature, the validity of the finite element (FE) model is demonstrated. The model effectively traces load-deformation behavior and failure mechanisms reported in reference tests. Specimens are compared with respect to variable parameters such as aspect ratios (1.3, 1.6, 2) and RAC replacement ratios (0%, 50%, 100%). A parametric study is conducted with variable RAC replacement ratios and aspect ratios. The outcomes predicted by the model and the test results are found to be consistent. Failure initiation is found to be significantly influenced by a higher aspect ratio. The key findings, recommendations, and future research requirements, emphasizing the practical implications for the design and construction of rectangular RACFST columns are summarized.