The utility model relates to a connecting structure of a steal beam and a composite floor slab of a building, and belongs to the field of building components. The steel beam is connected with the composite floor slab. The connecting structure is characterized in that the composite floor slab is composed of a prefabricated plate and cast-in-situ concrete, prestressed steel bars and light steel ribs connected with the steel beam in one direction or double directions are embedded in the lower prefabricated plate, oblong holes are formed in the end portions of the light steel ribs, the upper surface of the prefabricated plate of the composite floor slab is uneven, notches are formed in the upper portions of the light steel ribs, and the insides of the notches are paved with upper steel bars of the composite floor slab; a connecting plate and reinforcing steel ribs connected with the composite floor slab are welded on an upper flange plate of the steel beam, the reinforcing steel bars are arranged on the upper end portion of the prefabricated plate in a paving mode, oblong holes are formed in the connecting plate, the composite floor slab is arranged on the upper flange plate of the steel beam, bolts penetrate through the oblong holes in the end portions of the light steel ribs and the oblong holes in the connecting plate welded to the upper portion of the upper flange plate of the steel beam for fixing, and the reinforcing steel bars welded on the upper flange plate of the steel beam are connected with the composite floor slab; the cast-in-situ concrete is located on the prefabricated plate to cover the light steel ribs, the upper steel bars and the reinforcing steel bars.