To improve the bending performance of long-span bamboo scrimber beams, a new method of prepressing-cold pressing-hot forming was used to produce steel-reinforced bamboo scrimber composite beams. For steel-RBSC beams, four-point bending experiments were performed to study the failure mode, bending performance, load-displacement relationships, and strain history curves of composite beams. Based on cohesive zone model theory, finite element models of steel-RBSC beams were established and the bonding stress of the adhesive layer in steel-RBSC beams revealed. The results indicated that the failure mode of composite beams was fracture failure of the bamboo scrimber at the bottom between the loading rollers. Steel reinforcement had a significant effect on the strength and stiffness of bamboo scrimber beams. The maximum static stiffness of composite beams was increased by 70.15%, compared with that of bamboo scrimber beams. The load-displacement curves, mechanical properties, and section strains predicted by finite element models were consistent with the test results, verifying that these finite element models were correct. The main influence factors for the peeling of the adhesive layer were normal stress, shear stress, and mixed stress in the areas of pure bending section, bending-shear section, and loading roll area, respectively.