Abstract: A quadrature demodulation method based on echo-frequency tracking is proposed to eliminate the flow velocity estimation error caused by ultrasound frequency downshift.First a linear model is established for the central frequency downshift along depth direction.Then the frequency downshifts along depth are estimated through autocorrelation technique based on the biased base-band signal obtained by conventional I/Q (Inphase/Quadrature) demodulation, and the estimated values are used for fitting the linear model.Finally the central frequencies are tracked by the fitted model,and used as the reference frequencies for I/Q demodulation.Simulation and experimental results show that nearly unbiased base-band signal can be obtained by the proposed demodulation method,so that the flow velocity estimation can be more accurate and the quality of ultrasound color flow imaging can be improved.