The power spectrum of the autodyne signal of CO2 laser was analysed by the high-speed data acquisition and processing system at 10 MHz. The power spectrum is distributed in the low-frequency region. The bandwidth of the autodyne signal is increasing with the Doppler frequency shift. The integral of the power spectrum for frequency is converged at the power of the autodyne signal. The experimental results show that the analytic method by high-speed data acquisition and processing has more advantages over the spectrum analyzer.