The slip between the steel girder and concrete slab may decrease the global stiffness of composite beams, which should be taken into account in static and dynamic analyses. In this paper, by the derivation of the fundamental dynamic theory, the analytical solution of deflection expression functions for simply-supported composite beams, which are conventionally solved by numerical methods, are derived from the direct balanced method of composite beam theory. After a series of mathematical transformations, the deflection expression functions finally meet series summation conditions, and then the analytical solution of deflection expression functions under concentrated loads are obtained by substituting the corresponding Bernoulli functions and numbers. The deflection expression functions can not only reflect the result of common beams, but also reflect the additional deflection caused by slip. The analyses results are compared with the experimental results, showing a good agreement. The results show that after considering the relative slip of the composite beams, the governing motion equations are more complex and show different dynamic characteristics comparing with common beams; and the static results can be counted as a special case of dynamic response.