In the modern war, the intelligence of conventional ammunition is of great significance. However, in the process of intelligent transformation, in the face of high overload, high speed test environment, the traditional inertial devices has been unable to meet the engineering requirements in many aspects.Such as the range, precision and cost. The geomagnetic field is a regular stable field, It has important applications in the fields of earth science, aeronautics and astronautics, resources exploration, transportation and communication, national defense construction, earthquake prediction and so on. According to the characteristics of the rotational speed of conventional ammunitionthe,geomagnetic data is collected by the combination of the geomagnetic sensor. The moment of zero crossing of geomagnetic signal is calculated by software algorithm,and then settle the projectile rotation speed. The feasibility of this method is proved by experiment.