Abstract: To solve the problem that the relative slip occurring on the interface between U-bar and concrete slab in a composite girder causes the redistribution of stress under external loading and then the bearing capacity of the girder is reduced with deformation increased, a deformation analysis based on energy theory is made to the composite girder. Based on energy variation principle, the differential equations of deformation are derived considering the effect of relative slip, with the calculation formulas of deformation of the composite girder given to analyze the examples under different conditions, thus discussing the influences of connection stiffness and the steel plate thickness of cold-formed U-bar on the deformation of composite girder. Calculated results showed that the deformation of the composite girder is distributed nonlinearly along its length, and decreases with increasing connection stiffness and steel plate thickness.