Tests have been carried out to study the mechanical property when the negative bending re-gion of combination of steelconcrete composite box beams is under compression.This paper empha-sizes the discussion of the computation model of the local stability when box beams is under compres-sion.By comparing the local stability of box beams when under compression with the box beams whichis welding studs,pouring concrete slab of certain thickness,this paper is to provide the reference forthe optimization design of the bottom slabs of the negative bending region of combination of steel-concrete composite box beams.Results showed that castinplace concrete slab in compressionstate can effectively prevent the occurrence of local buckling instability;it can reduce the stress of theconcrete slab and improve the negative bending region of concrete beams of the ultimate bearing ca-pacity.