Based on the background noise, the generation of short pulse series and comb-like spectrum are numerically simulated by the amplitude modulation of continuous wave (CW). The results show that the stability of short pulse series and comb-like spectrum can be improved by the amplitude modulation. The modulation parameters play a key role on the output signal-to-noise ratio(SNR) and the stability of comb-like spectrum. The region of modulation parameters with stable comb-like spectrum is also obtained.