Abstract: The interface slip will occur unavoidably in the steel-concrete composite girder with prestressed FRP sheets if the girder is under the external load, and the internal force of prestressed FRP sheets will gradually increase with the external load to redistribute the internal force of the girder so as to affect the bearing capacity and deformation of the girder. The differential equations of deformation are therefore derived on the basis of the theory of elasticity, considering the effect of the relative interface slip internal force increment of FRP sheets. And the calculation formulas of deformation are given for the composite girder under symmetrical point load. The calculated results show that the deformation of composite girder increases gradually with the interface slip, but the effect of internal force increment of the prestressed FRP sheets on the composite girder deformation is so small that can be ignored, thus giving the reduced calculation formulas of deformation for the steel-concrete composite girder with prestressed FRP sheets.