Theoretical improvements have been made in the unified Hauser-Feshbach and exciton model. The angular momentum conservation is considered in reaction processes for both equilibrium and preequilibrium mechanisms. The recoil effects in varied emission processes are taken into account strictly so the energy balance can be held exactly. A method for calculating double-differential cross sections of composite particles is proposed. Based on this theoretical frame, the UNF code (2001 version) for calculating neutron-induced reaction data of structural materials below 20 MeV was issued. The functions of the UNF code are introduced.