In order to meet the demand for large-span development of concrete composite slabs in recent years， a kind of composite slab filled with the lightweight aerated concrete block was proposed （i.e. CCS）. Comparison tests were conducted to study the bending performance of CCSs with different parameters （longitudinal rib， filler， transverse rib） and ordinary concrete cite-in-site slab. Analyses were conducted on the mechanical parameters including ultimate bearing capacity， bending stiffness， load-rebar strain curves， and anti-slip performance. The differences in flexural properties between cast-in-site slab and CCSs were compared and analyzed； the influence mechanism of different parameter changes on the bending stiffness of CCSs was mainly studied. On this basis， the short-term bending stiffness calculation formula with different stress stages was proposed. Results show that： CCSs and ordinary concrete cite-in-site slabs share similar flexural behaviors， show evident features before failure， and their crack distribution is also nearly the same， presenting the typical bending failure mode of one-way slabs. The cracking load of the cast-in-site slab was higher than that of CCS， but the yield load and ultimate bearing capacity were lower than that of CCS. The constructional patterns of CCS have some impact on its flexural performance. According to the tests in this paper， the influencing degrees from large to small are as follow： longitudinal rib > whether filler block was exposed > transverse rib.