In order to obtain the harmonic plateau, contributed by single harmonic emission peak, an effective method to control the harmonic spectrum by using the negative chirped pulse combined with an ultraviolet pulse has been proposed. It is shown that with the control of the negative chirped pulse, not only the harmonic cutoff can achieve the referenced value, but also the harmonic plateau is coming from the single harmonic emission peak. Further, with the proper introduction of an ultraviolet pulse, the intensity of harmonic plateau can be enhanced and reach the referenced value due to the ultraviolet-resonance-ionization. Finally, an ultra-short 36as pulse can be obtained by superposing the harmonics on the plateau. The present investigation provides a new method to obtain the attosecond pulse by using the low-intensity combined field, which is helpful to the development of the laser source.