Dual-frequency laser coherent detection technology is used to perform the Doppler measurement of high-speed targets. The power spectral function of the electric signal for the dual-frequency laser coherent detection with intensity disturbance can be obtained by combining the random statistical theory with Wiener-Khintchine theorem. Based on the theoretical model, the influences of beat line width and disturbance frequency on the signal power spectrum is analyzed, and the numerical simulation and experiments are conducted. The results reveal a Lorentz-type power spectrum distribution. For long-distance detection, an increase in the line width of the beat signal broadens the power spectrum. When the intensity disturbance frequency is close to the Doppler frequency shift, the power spectrum broadens and the signal amplitude decreases. In this case, the spectrum extraction process becomes more difficult, and the Doppler measurement accuracy decreases.