Aiming at the dual-beat-frequency light which is obtained by using fiber delayed self-heterodyne system, the theoretical model for power-spectrum of beat-frequency signal influenced by intensity fluctuation and phase noise of laser source is built. Based on the model, the numerical analyses and simulations are carried out. The results show that the short delay time between the two paths of laser can better decrease the influence of laser noise than the long delay time. When the delay time is shorter than the coherence time of the laser, the signal to noise ratio (SNR) of the beat-frequency signal can decrease as the laser linewidth and the intensity fluctuation increase. When the delay time is much longer than the coherence time of the laser, the laser phase noise becomes the main reason for spectral broadening of the beat-frequency signal. In addition, the power of the beat-frequency signal can decrease as the phase noise increases. The intensity fluctuations also make contribution to the broadening of the beat-frequency signal while increasing the power of the beat-frequency signal. When the delay time is long, the linewidth and power of the beat-frequency signal will change in accordance with the law of cosine as the delay time changes, and there are optimum operating points.