To produce the harmonic spectra with high conversion efficiency and high photon energy, the effect of the multi-color chirp waveform on the harmonic spectra was theoretically studied by solving the Schrdinger equation. The results show that, under the fixed laser intensity, the harmonic cutoff energy can be effectively extended by the best 3-color chirp waveform. The harmonic intensity can be enhanced by the best 4-color chirp waveform. Finally, the isolated attosecond pulses of 42as can be obtained by superposition of the harmonic spectrum of the best 3-color and 4-color combined waveforms. The results are helpful for the generation of ultra-short attosecond pulses.