Dynamic flow measurement is becoming more and more important in the process of research and development of production. In order to test the dynamic response characteristics of the flowmeter, the dynamic measurement of the piston displacement of the active piston prover is carried out by using the laser interferometer, and the step characteristics of the prover are analyzed. The test results show that the time constant is between 7 ~ 60 ms in a certain step flow range. It is suggested that the transmission structure and servo control parameters of the prover be further optimized to improve the step characteristics of the device.