Abstract: Slips at the interface of steel girders and concrete slabs have significant effects on the mechanical behavior of composite beams in elastic stage. Based on the differential equations of slips, the solutions of slip and slip strain in negative bending regions of continuous composite beams are obtained. By means of the slip strain solutions, the strain distribution of composite sections under negative bending is analyzed, and the methods for calculating the load-carrying capacity of composite beams are proposed. It is shown that the additional curvature induced by slip effect decreases the elastic moment strength of composite beams under negative bending moment in most conditions. This partial interaction effect should be considered in design.