Abstract: Under the action of bearing load, the relative slip at interface between steel beam and concrete of a composite beam affects greatly its bearing capacity, deformation and resistance to shock. In order to evaluate correctly the load-bearing performance of a prestressed steel composite beam with high-strength concrete, the equivalent cross-sectional area and virtual work principle are taken into account to derive the calculation formulas of deformation of the composite beam according to actual loading conditions, including all interactions between different forces and the effect of slip at interface. Analyzes the effect of relative slip at interface on the deformation of the composite beam under symmetric concentrated load and/or uniform load. The calculated result is verified in good conformity with test result, and shows that the slip at interface can increase the deformation of the composite beams by over 10 percent.