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The clamp-on ultrasonic flowmeter is widely used in the testing fields of petroleum transmission, flow tracking, water supply and drainage because of its advantages such as no need to damage the pipeline, easy installation, and low maintenance cost. A clamp-on liquid ultrasonic flow detection system based on the time difference method is designed. It adopts a system architecture combining FPGA and single-chip microcomputer. The single-chip microcomputer is responsible for data processing, display and output, and FPGA is responsible for logic control and providing drive signals for hardware circuits. TDC -GP22 high-precision timing chip is used to measure the transit time of ultrasound. DAC circuit is used to realize variable discrimination signal reference technology. Finally, an external clamp-type ultrasonic flowmeter test platform was built. The test results show that the developed prototype effectively improves the test accuracy of the ultrasonic flowmeter, with an error of less than 4% in the laminar flow zone and less than 2% in the turbulent flow zone.