The muzzle velocity measuring coil is mostly adopted in the velocity measuring method of timing and ranging rifle cannon weapon system, but the shortcomings of the method result in system complexity, high cost, easy interference to induction setting signal. Therefore, the mature technology of magnetic sensor was introduced in fuze design. After the projectile went out of the muzzle, the projectile rotating period was repeatedly measured by the geomagnetic sensor, and the single chip microcomputer and ultra high speed A/D in the fuze circuit was used to perform the operations of signal collection, data processing and trajectory calculation to convert the average rotating period of the projectile to actual initial velocity. The principles utilized in this paper can simplify the weapon system and decrease the cost and innovation difficulty in in-service air defense weapon system.