During correctional control of trajectory correction projectiles, an accurately real-time measurement of projectile's roll angle is necessary. A math model of combined measurement of projectile's roll angle was established by using the principle of combined measurement of geomagnetic and satellite, also, the singular point position and error analytic formula was derived. The singular point position of combined measurement was verified by using numerical simulation method, and the main factors of system error for the combined measurement were found out to be pitching angle and yaw angle error. After removing the singular point area, and the area where the ballistic angle of attack and sideslip angle are less than 1°, the projectile's roll error is less than 5° by the combined measurement of geomagnetic and satellite.