Grid and inverse C-shaped reinforced flat slab with weight-reducing cavity

The invention relates to a forming method of a grid and inverse C-shaped cavity flat slab. The flat slab is composed of a light and bubbling precast slab, reinforcing steel bars and concrete. The flat slab is formed into a tight structure after being combined to the cast-in-place structure and the two mutually cooperate with each other, so that the weight of the slab is reduced by 40% and a large-span flat slab is able to be formed and cast in place. The cross section of the slab is an inverse C-shaped light cavity slab structure, which is also an I-shaped structure, so that the rigidity of the slab is reinforced and the slab is applicable to buildings with an open-space structure, such as a commercial building, the library, a teaching building, an office building, a bus station, a park, an amusement facility and civil buildings. The flat slab is also applicable to multi-layer or single-layer industrial factory buildings, garage and other industrial applications.