Attitude measurement based on magnetic field is a hot spot today and provides technology support for the roll angle measurement, especially in the application on trajectory correction projectiles. A method with the artificial magnetic field installed on gun muzzle was provided and it provides the reference angle for the roll attitude measurement of projectile. And the magnetoresistive sensors are used to measure the magnetic field to offer the roll attitude. The words pay more attention to the fundamental theory and operational principle of the roll attitude measurement, also the system design project. The system can give real-time roll attitude, and the measurement is simple and available.