Abstract: The mechanical performance of tightly jointed seams is directly related to the safety of composite slab structures, but the mechanism by which their construction parameters influence the flexural behavior of composite slabs remains unclear. Therefore, this study aims to systematically investigate the influence mechanism and patterns of joint configuration parameters on the flexural behavior of tightly jointed reinforced concrete composite slabs. A refined finite element model of the structure was established using the ABAQUS finite element analysis software. By comparing the numerical simulation results with experimental load-displacement curves, crack development patterns, and failure modes, the accuracy and reliability of the model in predicting the flexural performance of composite slabs were verified. Based on this validated model, a parametric analysis was conducted. The results show that when the diameter of the additional reinforcement is increased from 8 mm to 10 mm, the ultimate load increases by 23.4%; however, a further increase to 12 mm does not result in a significant improvement. Reducing the distance between the truss bars and the joint from 300 mm to 100 mm increases the ultimate load and ultimate displacement by 16.0% and 12.6%, respectively. Increasing the friction coefficient at the interface between new and old concrete also enhances the overall stiffness and load-bearing capacity. In practical engineering, the flexural performance of tightly jointed composite slabs can be significantly improved by appropriately increasing the reinforcement in the joint region, arranging truss bars closer to the joint, and ensuring sufficient surface roughness at the composite interface.