Based on the full quantum theory of high-order harmonic generation (HOHG) (Lewenstein et al 1994 Phys. Rev. A 49 2117-32), we study the effects of the static electric field on HOHG. It is found that the presence of the static electric field breaks the inversion and reflection symmetry, as a result, the HOHG spectrum exhibits a double-plateau structure. Furthermore, the cut-off of the HOHG no longer corresponds to the maximum kinetic energy the ionized electron can obtain from the laser field. Instead, it corresponds to the maximum kinetic energy of the electron with one return. Finally, for ultra-high static electric field, we find a monotonic decrease of the harmonic intensity as the harmonic order is increased.