High-speed gas flow measurement, such as flare gas flow, is of great significance for energy saving and emission reduction, and for improving enterprise economic benefits. Ultrasonic flowmeter has a wide range, stable measurement, and is not affected by component changes. At present, ultrasonic flowmeter is the mainstream product of torch gas flow measurement. At present, this kind of products are basically foreign monopoly. The wave propagation time of domestic ultrasonic flowmeter fluctuates greatly in the measurement of high-speed gas, resulting in a large error of measurement results. In order to find a suitable algorithm for estimating the propagation time of sound waves, ultrasonic signals at gas flow rates ranging from 10 m/s to 70 m/s were collected from professional metering devices of instrument manufacturers. Four signal processing methods including envelope fitting method, autocorrelation method, cross-correlation method and wavelet time-frequency analysis method were studied. The experimental results show that wavelet time-frequency analysis has a good time of fly estimation effect. Standard deviation is smaller than 9 μs. But there are still fluctuations at high speeds. Averaging, least square method or secant slope method can eliminate data fluctuations and improve measurement accuracy and stability.