Study on the dynamic excitation is the basis of dynamic response analysis of gearbox. In the paper, the dynamic coupled model of the star gearing was established. By analyzing the casing substructure and extracting the coherency nodes, the gearing and casing was coupled. On the basis of analyzing dynamic meshing stiffness and transmission error, the dynamic meshing force of the star gearing was analyzed and calculated. By converting the dynamic meshing force, the dynamic force on the bearing was obtained. The dynamic excitation for dynamic response analysis of casing was achieved by applying the load on the coherency nodes. The method used in the paper has provided proper dynamic excitation for the dynamic response analysis of casing.