Currently, rotational speed is commonly measured by radar and optical high-speed photography method, but their high cost and failure in real-time data display result in falling back on the method for rotational speed measurement. Therefore, internal measurement was designed to test projectile speed. Test devices are installed inside projectile, and rotational speed change could be reflected by induced electromotive force produced by diaphragmatic geomagnetic sensors cutting magnetic induction line, and a new method for processing speed was proposed, which was center point method. Through several experiments, feasibility of the system was verified. Compared with conventional method, the measured result was small in error with high precision, and this could be referred for future research on projectile nutation parameter.