Regarding the issue of how to precisely measure the attitude information of the projectile body of guided projectiles during trajectory correction, a solution that combines a magnetometer and a gyroscope for attitude measurement is proposed. The initial attitude of the projectile body is resolved by comparing the output information of the on-board magnetometer with the standard geomagnetic field information. Considering that the high overload and high impact during the launch of the projectile might lead to abnormal gyroscope signals, to enhance the adaptability of the system, the Kalman filtering algorithm and the Interactive Multiple Model (IMM) algorithm are integrated to achieve the data fusion of the output information of the magnetometer and the gyroscope, thereby realizing the subsequent update of the projectile attitude. The flight experiment of the projectile validates the combined attitude measurement scheme, and the results indicate that this method can accurately obtain the attitude of the projectile.