As a projectile's attitude measurement is significant for flight control, the 'crest and trough method' only based on the geomagnetic sensor signal is proposed to improve the angular measurement accuracy. Firstly, the geomagnetic sensor distribution is introduced, then the mathematical deduction is expatiated and the procedure of the 'crest and trough method' is elaborated. Based on the drawbacks of existing methods, the proposed method focuses on improvements that are vital to enhance the measurement accuracy. Finally, simulation and experiment are carried out to prove the effectiveness of the proposed algorithm, and the results used to demonstrate that this method not only has no cumulative error, which occurs in micro-inertial sensors, but also has better precision than traditional geomagnetic angular measurement methods because of the twice faster angle update rate and elimination of interpolation.