Solving the time-dependent Schrodinger equation for a one-dimensional atom in an intense laser field based on the basis function expansion and a linear-least-squares fitting method, we examine the influence of the bound states,-continuum states and the interference between them of the model atom on the high harmonic generation. It is found that the main contributions to the low-energy harmonic generations come from the bound-bound interaction, while the high-energy harmonic generations are from the bound-continuum interaction. Beyond the cut-off, the interference of the bound-bound and bound-continuum interactions leads to the disappearance of the harmonic generation.