To improve the mechanical performance of large-size concrete-filled steel tubes without increasing the steel consumption, this study conducted a numerical analysis of the flexural behavior of concrete-filled square steel tubes with embedded steel angles and spiral stirrups (CFSST-SASS). A finite element (FE) model was established and validated against experimental results. Considering the composite confinement effects from the steel tube, angles, and stirrups, a compressive constitutive model for the core concrete of CFSST-SASS was developed. The flexural working mechanism and composite action were analyzed. Parametric studies were carried out to investigate the influences of steel ratio, volumetric stirrup ratio, angle configuration ratio, confinement diameter-to-width ratio, and material strengths on flexural capacity, stiffness, and ductility, with recommended ranges proposed. Results show that CFSST-SASS exhibits favorable composite action and flexural performance. The steel ratio, steel tube strength, and concrete strength are the primary influencing parameters. Increasing the angle configuration ratio can improve the bearing capacity and ductility. Recommended ranges are: steel ratio 5.96–11.61%, steel tube yield strength 355–690 MPa, concrete cubic strength 30–70 MPa, volumetric stirrup ratio 0.6–1.5%, angle configuration ratio not exceeding 2.5%, and confinement diameter-to-width ratio 0.5–0.9. Predictions from current codes are conservative. Based on the unified theory, a formula for the flexural capacity of CFSST-SASS was proposed, which shows good accuracy, with errors within 5%, verified by numerous numerical results.