Due to the muzzle aftereffect, the projectile will have complex attitude motion caused by the nutation effect of gyroscopic precession, which makes it difficult to calculate the initial roll Angle at the muzzle by the missile-borne geomagnetic sensor. In view of the above problems, combined with the ballistic characteristics of the rising stage of the rotating projectile, this paper proposes a method of using the geomagnetic sensor of the three-axis projectile to reverse the initial roll Angle: Assuming that the projectile has no yaw motion, the relative roll Angle variation is fitted by the magnetic measurement data of the disturbed section, and the bending pitch Angle of the projectile is solved by using the magnetic measurement data of the smooth flight section, and the Euler Angle relationship is established. Numerical simulation results show that in 6-DOF ballistic simulation, when the initial nutation/precession amplitude is 4° and there is only sensor measurement error, the calculation error of the proposed method is about 0.3°, and when the yaw Angle error is considered to be 5°, the calculation error of the proposed method is less than 2°. The actual shooting test results show that the error of this method is about 2°.