Based on the measurement method developed to integrate the magneto-resistive sensor and global positioning system (GPS), the blind spot of magnetic measurement during the combined measurement is analyzed. In the measurement of projectile roll attitude, the possible blind spot of magnetic measurement exists, and then the roll attitude calculation becomes inaccurate. Aiming to solve this problem, an idea of using the roll angle to the velocity vector instead of the roll attitude output is put forward, and a model is proposed to calculate the roll angle to the velocity vector axis according to the GPS output, which is used as compensation to the inaccurate roll attitude. Moreover, the feasibility of the compensation is demonstrated. The proposed model is verified using numerical simulations, and can overcome the blind area drawback of the magneto-resistive sensor effectively, which illustrates that the accuracy of the roll attitude measurement is improved.