The measurement and control system based on permanent magnet ac generator, which performs functions of executive steering gear and load power, was designed to make accurate trajectory correction for the double-spinning projectile. A calculation model of the rudder's rolling Angle in inertial system was also established. A new method combining software and hardware filtering and compensation was put forward in order to reduce the strong magnetic interference of the steering gear on the geomagnetic sensor. A dual closed-loop control system architecture which combines the incremental PID control strategy was adopted. The generation voltage is in the range of 10 ~ 90.6 V when the steering gear is in the range of 80~500 Hz; The rotational speed measuring precision of the projectile and rudder is better than 0.1 Hz, and the rudder position measuring precision is better than 1°. Shooting range experiment results prove that the system response time of speed control is better than 0.1 s, and the stability of position control is better than 5°. Conclusions show that the steering gear system has good feasibility and reliability underground and missile-borne conditions.