The Photon Doppler Velocimeter (PDV) is a non-contact velocity measurement equipment with high accuracy and high-time resolution, which can obtain the continuous interior ballistic velocity of ultra-high-speed launchers. Continuous velocity data is very important for ultra-high-speed experiments. It can be used to understand the performance of ultra-high-speed launchers and the physical processes of ultra-high-speed, as well as to develop the theory of interior ballistics. Limited by the small size of the muzzle, the serious attenuation of laser energy and (the limitation of) the bandwidth of detector, it is difficult for ordinary PDV to obtain continuous ultra-high-speed interior ballistic velocity. In this paper, we have developed a large depth field PDV with an effective working distance greater than 7 m, which is constructed based on fiber Mach-Zehnder interferometer. The emission aperture of optical antenna is 25 mm, the beam waist of emission position is located at 3.3~3.4 m, and the diameter of beam waist is 1 245 mm. In order to verify the performance of the system, we first simulated the high-speed motion process by using a rotating turntable and a motor, and tested the measurement error of the PDV system. In the velocity range of 1~40 m/s, the measurement uncertainty of the PDV can be controlled at 2.48%. Then we carried out experiments on the ultra-high-speed ballistic target (FD-18A) of China Aerodynamics Research and Development Center (CARDC), and repeatedly obtained the continuous ultra-high-speed inner ballistic velocity of the ultra-high-speed two-stage light gas guns. In the experiments, we placed a reflector directly behind the muzzle to change the direction of the laser signal and put the optical antenna on one side of the reflector. Finally, the PDV recorded the velocity changes of the launch model from static acceleration to about 2 km/s and 7 km/s, with the maximum velocity of 6.89 km/s. By comparing with the numerical simulation results, it is found that the measured velocity of experiment is lower than the simulation velocity in the test with a velocity of 2 km/s. While the measured velocity of experiment is higher than the simulation speed in the test with a velocity of 7 km/s, and the deviations are -20.11%, -23.7% and +9.15%, respectively. Through the analysis of velocity-acceleration data, it is found that the difference in friction between simulation and experiment may be the main reason for the difference of velocity. The actual friction force of the ultra-high-speed projectile in the ballistic target is greater than the theoretical friction force given in the simulation, so it may cause that the maximum speeds and accelerations are lower than the theoretical results in the test with an estimated launch velocity of 2 km/s. In the test with a velocity of 7 km/s, the mass of the projectile decreases rapidly due to severe friction, so the maximum velocity and acceleration in the second half of the movement are gradually larger than the simulation results.