Abstract: The large static drift will deteriorate dynamic measurement accuracy and precision of the time difference method ultrasonic flowmeter, while accurate determination of the received ultrasonic signal can efficiently reduce the drift of the static instantaneous flow. The traditional hardware threshold model is relatively weak in anti-interference, and there is large static drift, which seriously affects the measurement precision improvement. In this paper, the zero threshold smoothing model is improved a lot on the threshold model so that it can accurately determinate the received ultrasonic signal. The measured results prove that this method can efficiently reduce the drift of the static instantaneous flow, and improve the precision of the time difference method ultrasonic flowmeter.