In order to enhance the intensity of high order harmonic spectrum, a method of improving harmonic intensity by using resonance ionization is proposed. The results show that, in case of He atom system, the harmonic intensity can be enhanced by 50 times under the influence of ultraviolet resonance ionization. Moreover, with the introduction of chirped pulse, the harmonic cutoff can also be extended, showing an intense and broad harmonic plateau. In case of H+2 molecule system, the harmonic intensity canbe enhanced by nearly 20 times under the influence of charge resonance enhanced ionization. And with the optimization of the half-cycle unipolar pulse, the extension of harmonic cutoff for H+2 molecule system can also be achieved, showing a harmonic plateau contributed by single harmonic emission peak. The further study shows that by superposing some harmonics on the harmonic plateaus, the attosecond pulses with durations of 36 as and 32 as can be obtained for the two systems, respectively. It is confirmed that the proposed scheme is helpful to enhance the intensity of attosecond pulse.