In order to investigate the mechanical performance and failure modes of laterally connected precast hollow core composite slabs with board-side dense splicing, static model bending performance tests were conducted on three tightly connected hollow composite slabs and one seamless hollow composite slab under monotonic loading. The failure mode, crack distribution, bending bearing capacity, and bending stiffness of additional steel bars at the joints of the board were analyzed. The results indicate that, under normal operating conditions, the flexural performance development of concrete hollow composite slabs with tightly connected panel sides is generally consistent with that of the non-spliced cast hollow composite slab. Under ultimate conditions, tearing or brittle fracture failure at the joint interface is prone to occur in the composite slabs, resulting in a reduced flexural bearing capacity. With the increase joints, the flexural capacity of the hollow composite slab is decreased by 17.1%; while positioning the joints away from the most unfavorable stress section, the flexural stiffness of the section will be improved by 13.5%. Single-seam tightly connected concrete hollow composite slabs on the panel sides meet the design requirements for bidirectional stress performance, effectively enhancing construction efficiency, shortening construction periods, and reducing construction costs.