By solving the time-dependent Schrodinger equation with a one-dimensional short range potential, the effect of initial population on the high harmonics generation was studied. The results reveal that when a moderate intensity laser acts on an atom or ion of high ionizational potentials, if the initial population is on the ground state, the atom or ion will not be ionized, only lower harmonics can be obtained. If the initial population is partily on the ground state and partly on the excited states, the atom will be ionized partily, and the lower harmonics will be weak and the high harmonics be increased. If the initial population is on the excited states, the atom will be ionized completely, and the orders and the conversion efficiency will be lower. These results show that partial ionization of the atom will enhance the harmonic generation, and the two-step semiclassical model is confirmed to some extent.