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Aiming at the problems that the traditional liquid flowmeter sensor will damage the pipeline or hinder the flow of liquid, and the operation and installation are more complicated, a portable non-destructive ultrasonic flow test system based on the time difference method is designed. The system adopts the form of external clamped ultrasonic probe, which can realize the accurate measurement of liquid flow without destroying the pipeline system. This system uses a measurement algorithm that combines coarse time and fine time measurement, and designs a FPGA high-speed, high-precision time measurement algorithm circuit based on delay line interpolation, which can achieve a measurement rate of up to 1050 Hz. A signal conditioning and calibration circuit is also designed, which has strong positive and negative gain adjustability and high signal-to-noise ratio output capability. The gain adjustable range is -23.5 dB ~ +116.5 dB. A variety of special mounting rails for sensors are also designed to ensure its installation accuracy. Finally, a verification test was carried out in the metrology laboratory, and the result showed that the designed test system meets the technical requirements of JJG1030-2007 with an accuracy of 0.5 level. The accuracy is less than ±0.5%, and the repeatability is less than 0.1%.