Modular steel buildings were characterized by the off-site prefabrication of standardized volumetric units, offering the significant advantages in construction speed, environmental sustainability, and on-site safety. It was well-known that the highly integrated buildings was prone to the global collapse assembled by only column-to-column connections and the beam-to-beam connections could greatly promote the overall mechanical performance. However, the cooperative bending performance have not been fully understood in a theoretical prospective, thereby unable to conduct the performance-based structural design in practical engineering. In present study, the superimposed bending performance of laminated double beams in modular steel buildings were theoretically and experimentally investigated. The theoretical analysis was performed on the modular laminated beams with friction and point connection of high-strength bolts. The analytical expressions for interfacial slipping strain were established based on the fundamental differential equation. The theoretical models of interfacial slippage were derived for the laminated steel beams with different connections. Then, the mathematical expressions were established for the bending curvature of laminated beams, considering interfacial slippages. In this way, the theoretical procedure to calculate the equivalent initial bending stiffness were ultimately developed. By this way, the mechanical performance of laminated beams was analyzed and the superimposed bending effect was further evaluated. It was indicated that the bending capacities of laminated beams with bolt connections were approximately improved by 8% compared to specimens with only friction restraints. In addition, the enhancement of initial bending stiffness reached 17% to 28%. Finally, the prediction models of the initial bending stiffness were greatly validated by the experimental data, which could provide the theoretical foundations for the structural design of laminated beams in modular steel buildings.