Abstract: The reinforced concrete superimposed slabs with steel bar truss are composed of prefabricated concrete bottom slab, steel bar truss and post-cast-in-situ composite layer. Concrete cast-in-place belts are set between prefabricated slabs, and the longitudinal reinforcements are lapped and connected at the cast-in-place belt to form an integral floor slab. To study the fire behavior of laminated slabs under high temperature-load coupling effect, fire-resistance tests are performed on four sides simple supported superimposed slabs with different thickness of post-casting layer and assembling modes. The results show that under the high temperature-load coupling effect, there is no obvious separation between the prefabricated bottom plate and the laminated layer, and the laminated plate still has good overall working performance during the fire. Longitudinal cracks appear on the surface of the slab along the joint surface between the cast-in-situ belt and the prefabricated slab and along the steel truss. The final joint surface forms through cracks, and the floor slab is burned through and reaches the fire resistance limit. The average fire resistance limit of the tested board is up to 3 hours. During the fire exposure, the concrete at the bottom of the cast-in-situ slab spalls seriously, and the steel bars connected to the lap joint at the cast-in-situ zone are directly exposed to fire and lost their effect, which greatly reduces the bearing capacity of the one-way slab in the same direction, and the residual bearing capacity of the slab should be calculated according to the one-way slab under the fire exposure. Due to the small thickness of the post-casting layer and the large width of the prefabricated slab with few seams, the fire resistance limit is shorter. The steel truss plays a key role in ensuring that the laminated layer and the prefabricated layer work together under the fire exposure.