The invention discloses a method and device for controlling the thickness of a cast-in-place reinforced concrete floor slab. When the reinforced concrete floor slab is prepared to be cast in place after steel bars of the concrete floor slab are bound, the device for controlling the thickness of the cast-in-place reinforced concrete floor slab is fixed on a floor slab bottom formwork used for casting the reinforced concrete floor slab in advance, the device for controlling the thickness of the cast-in-place reinforced concrete floor slab comprises a steel sheet, an inner rod and an outer rod, wherein the steel sheet is fixed on the upper end surface of the inner rod, concrete is poured into the floor slab bottom formwork until the surface of the poured reinforced concrete floor slab is flush with the upper surface of the steel sheet, and in this way, it is determined that the thickness of the poured reinforced concrete floor slab reaches the required thickness; and after the poured reinforced concrete floor slab is solidified, a fixing nut can be loosened and the outer rod and the floor slab bottom formwork can be removed. The method and device not only can achieve the purposes of controlling the thickness of the slab and preventing leakage, but also have the advantages that a screw out of the floor slab does not need to be cut, the construction is simple, rapid and labor-saving, and resource saving is achieved.