In order to improve the complex fluid flow measurement accuracy by ultrasonic Doppler method and to solve frequency complexity of the fluid ultrasound echoes, a mixed extraction algorithm for Doppler frequency difference value of echo is obtain to aim to the complex of ultrasonic echo for flow measurement. Based on the Fourier analysis theory, a hardware circuit is designed that is to obtain a signal representing the average frequency get an average frequency of the echo. And the significance of signal with this average is explained by Fourier transform algorithm. And then, the signal as an input to a digital frequency discriminator the Doppler frequency shift is to get. Basis of the design method above, an ultrasonic Doppler flow measurement system is designed. Experimental results show that the oil and water mixed fluid flow measurement error is less than 3%. Thus the validity of the frequency offset extraction method is confirmed.