Aiming at the problem that the research of ultrasonic measurement is still in the laboratory environment, a sensing system based on ultrasonic time difference method is developed in this paper, which can quickly measure the gas velocity in the environment with low flow field uniformity. The whole sensing system is controlled by FPGA, which greatly shortens the sampling calculation time of the sensing system and improves the response speed. The ability of the sensing system to cope with complex environments is improved by designing a rectifier. Finally, the physical experiment test shows that the rectifier greatly improves the measurement accuracy of the sensing system under the condition of the bend pipe. By verifying the measurement performance of the whole system at different temperatures, the ultrasonic gas flow velocity measurement system designed in this paper can measure the gas velocity in a large flow velocity range within 2.1ms, and the measurement error is not more than 4.5%.