Abstract: Liquid ultrasonic flowmeters have outstanding advantages in large flow rate measurement, but its proving is much more complex than other flowmeters, which limits its application. Four ultrasonic flowmeters were first used in the custody transfer system on FPSO in the South China Sea. We analyze the principle and features of ultrasonic flowmeters, and then design the proving scheme. We attempt to use pipe prover to prove ultrasonic flowmeters for the first time in China. By contrasting pipe prover and master meter proving data, we can draw a conclusion that increasing the standard volume of pipe prover and evaluating the meters with more proving runs can help to solve the proving problem of pipe prover, which is relevant for the other similar upcoming projects.