We theoretically investigate the high-order harmonic generation from stretched molecules in a linearly polarized intense field. By adopting an infrared pulse combined with an ultraviolet （UV） attosecond pulse, the ionization process can be controlled effectively. In this excitation scheme, the harmonic spectrum beyond Ip ＋ 3.17Up is significantly enhanced by two orders, where Ip and Up = e^2E0^2/（4mew^2） are the ionization and ponderomotive potential, then smooth broadband supercontinuum with the bandwidth of about 120 eV is obtained, which leads to an isolated sub-60- as attosecond pulse with a high signal-noise ratio. Moreover, the bandwidth of the supercontinuum is weakly dependent on the location and pulse duration of the UV pulse.
