Relative slip will occur at the interface of the steel girder and concrete slab when the composite beams subjected to external loads, which should be taken into account in the structural analyses. Based on the fundamental dynamic theory, the analytical deflection solution to the simply-supported composite beams under concentrated harmonic load is derived, and according to the differential relationships between the deflection and slip, the dynamic slip response is obtained, where the static component of the dynamic slip expression functions finally meets the series summation conditions after mathematical transformations. The theoretical results are compared with the experimental ones, showing a good agreement. The analysis results show that the static results can be essentially counted as a special case of dynamic response, and the harmonic component has a relatively small effect on the effective slip, but a great influence on the mechanical performance of the shear connectors.