In order to accurately calculate the deflection of steel-concrete composite girders under long-term loads，considering the shear deformation of composite girder，slip effect at the steel-concrete interface and shrinkage and creep of the concrete slab，the equilibrium differential equations for calculating the deflections of steel-concrete composite girder are derived by using the energy variational method. By introducing the boundary conditions on the simply supported and continuous composite girders under uniformly distributed load，the deflection calculation formulas can be obtained. The reliability of the calculation formulas is verified by comparing the experimental results and the results of finite element analyses. The results show that maximum deflection of two-span continuous composite girder considering shear deformation and interlayer slip increases by 37.4% when compared with the results from primary beam theory，and the deflection increases by 58% after considering the shrinkage and creep of concrete. The calculated maximum deflection of simply supported composite girder after considering the shrinkage and creep of concrete is 1.55 times larger than that of the primary beam theory. It can be seen that shrinkage and creep can have a significant influence on the girder's deflection. The research results can provide a theoretical basis for the deflection calculation of steel-concrete composite girders under the long-term loads.