To investigate the flexural performance of twin-welded T-shaped steel–concrete composite beams connected with perfobond rib (PBL) shear connectors, a three-dimensional nonlinear finite element model was developed using ABAQUS. The simulation results were compared with experimental data to validate the reliability of the model. Upon validation, a parametric study was conducted to assess the influence of structural parameters—including concrete strength, flange slab width and thickness, steel plate thickness and strength, shear connector spacing and hole diameter, and through-bar diameter—on the flexural behavior of the composite beam. Furthermore, a simplified plastic theory was employed to establish a flexural capacity calculation formula for the composite system. The results indicated that the proposed finite element model accurately captured the mechanical response and failure modes observed during the experimental tests. When the width, thickness of the concrete flange slab and the thickness of the steel plate were increased by 25%, 75%, and 100%, the ultimate flexural capacity was enhanced by 12.2%, 30.8%, and 41.3%, respectively. Increasing the concrete strength from C35 to C50 led to a 7.5% improvement in ultimate flexural capacity, while upgrading the steel grade from Q235 to Q420 resulted in a 71% increase. Enlarging the shear connector spacing from 100 mm to 250 mm caused a 6.5% reduction in flexural capacity, whereas the connector hole diameter was found to have a negligible effect. Increasing the diameter of the through reinforcement bars from 6 mm to 12 mm yielded a 2.2% improvement in capacity. The proposed flexural capacity formula showed good agreement with both experimental and numerical results.