The research of hypersonic process is limited by the transition model, the turbulence model, and the computational complexity. At present the tunnel test is still a better way to investigate the hypersonic process than the computational fluid dynamic (CFD) method. In this work, the principle and structure of all-fiber photon Doppler velocimeter (PDV) are introduced. The PDV is based on laser Doppler effect and consists of an all-fiber Mach Zehnder interferometer and an optical antenna. The measurement accuracy and distance of PDV are tested, showing that the error can be controlled to be within 0.17 m/s when the velocity of piston is below 40 m/s. At the same time, the measured distance of PDV can reach 26.3 m. The continuous velocity of the free piston is measured by using the PDV in high enthalpy expansion tunnel of China aerodynamics research and development center (CARDC). During the tunnel tests, the PDV is placed next to the tunnel, and the optical antenna is installed behind the observation window of the tunnel and connected to a circulator by optical fiber. When the driving pressure of the tunnel is 1.3 MPa, the maximum numerical simulation velocity of the piston is 88.981 m/s, and the velocity is measured to be 88.810 m/s with a relative deviation of –0.19%; when the driving pressure of the tunnel is 2.7 MPa, the maximum numerical simulation velocity of the piston is 125.100 m/s, the measured velocity is 123.843 m/s, and the relative deviation is –1.00%. The piston reaches a maximum velocity before impacting on the tunnel, and then decelerates rapidly under the joint action of the driving pressure and compression wave. Therefore, the soft landing of the piston proves feasible, which is put forward by Stallkerin the 1960s. Finally, the reasons why PDV missed the impact velocity of piston is discussed. Through the analysis of received intensity, it is believed that the absorption, scattering and expansion of the laser beam caused by the strong driving pressure is the main reason.