Abstract: 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 Schrödinger 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.