Tri-axial magnetometers used to calculate the heading information of vehicles. In this paper, a method of determining attitude variations using only one tri-axial magnetometer was proposed. The condition is that the vehicle has only four degrees of freedom (moving pitch and roll). In addition, the attitude changing process was decomposed. Therefore, the attitude was calculated with the data of current moment and precious moment. In this way, the affection of the third-axis turbulence was suppressed. Then, experimental results verified the accuracy of proposed method.