As one of the most typical non-contact velocity measurement methods, the laser interdiction method has the advantages of simplicity and reliability, and is widely used in the projectile velocity measurement of light gas guns and electromagnetic ejectors. Aiming at the influence of the unfavorable factors such as the small laser interruption distance and the short passing time of the projectile in the low projectile velocity and compact electromagnetic projectile launching system on the accuracy of the projectile velocity measurement, the length measurement error correction method based on the online measurement of the interruption distance of the coordinate machine and the time measurement error correction method based on the measured optical fiber and photoelectric conversion consistency deviation are proposed. The results show that the correction of projectile velocity is 0.69% based on length measurement error correction. Based on the correction of time measurement error, within the range of projectile velocity, the correction of projectile velocity is 1.51%~12.02%. In the electromagnetic ejection device developed, the measurement of projectile velocity and the evaluation of its uncertainty have been successfully completed by using the above methods.