The uniform‐load fire behaviors of four full‐scale integral composite beams with superposed slabs were tested, considering the post‐cast‐slab thickness, shear‐connector spacing, and precast‐slab overlap length on the steel‐beam upper flange. The concrete temperature distribution along the slab thickness, the temperature of the rebar inside the slab, the temperature distribution along the steel‐beam height, the bending stiffness, and the overall integrity of the superposed slabs and steel beam were studied. ABAQUS simulations of the temperature‐field and deformation behaviors of the composite beams with superposed slabs under coupled thermal and mechanical loading were performed. No damage to the post‐cast and precast slab bonding interface during the heating process was apparent. The post‐cast joint had no obvious influence on the structure, which could still bear loads. The composite‐beam temperature field and bending stiffness were not significantly affected by the bonding interface of the new and old concrete and the post‐cast joint. Overall, the post‐cast slab thickness and shear‐connector spacing predominantly influenced the composite‐beam fire resistance. The experiment and numerical results agree well, verifying the numerical‐model validity and feasibility. [ABSTRACT FROM AUTHOR]