Aiming at use magnetic detection progress calculating for the projectile roll angle, the problem of the bias from the sampling data, put forward a new method of projectile roll angle using magnetic detection. Follow-up to the original data processing method for the reference voltage, analog amplification, correction and zero point of the oval study. In the design of geomagnetic acquisition system, the original output voltage processes amplify. ISL60007 provides the amplifier AMP04 1.25V reference voltage, and makes output values biased. The results show that the magnetic data can be ideal to achieve the purpose of calculating the roll angle.