Muzzle velocity is an very important parameter in the small caliber air defense flake system with timing fuze. The precise, instant measurement of the muzzle velocity of each round can greatly improve the weapon's air defense efficiency by adjusting the time constant according to its real muzzle velocity. The theory and implementation of the measuring system is introduced. As key point in the ystem, the error control steps are detailed to discuss especially about the magnet washer. Besides, to reduce time consummation for calculation, a novel arithmetic based on pre-calculated table was put forwarded. Experiments have shown that this muzzle velocity measuring system has the advantages of muzzle attachable, high precision and short process time.