In order to obtain intense and broad harmonic spectral plateau, by numerical solution of time-dependent Schrdinger equation, the half-cycle waveform control for producing the optimal half-cycle harmonic emission conditions was proposed by using multi-color combined field. The results show that, by controlling the chirps of two-color field, the optimal negative half-cycle waveform can be obtained; while, by controlling the chirp delay, the optimal positive half-cycle waveform can be produced. Driven by the above waveforms, the harmonic cutoffs can be extended. Further, with the introduction of ultraviolet pulse, due to the ultraviolet resonance enhanced ionization, the harmonic intensity can be enhanced. Furthermore, when the ultraviolet energy meets the single and two photon resonance enhanced ionization, the harmonic intensity is remarkably enhanced. With the decrease of ultraviolet photon energy, the enhancement of harmonic intensity decreases. Finally, the single attosecond pulses of sub-50as can be obtained by superposing harmonics on the harmonic spectral plateau. The results are helpful for the control of high-order harmonic generation and the production of attosecond pulses.