The total-field magnetic anomaly is an approximation of the projection of the magnetic anomaly vector onto the normal geomagnetic field . However, for highly magnetic sources, the approximation error of cannot be ignored. To reduce the error, we have developed a method for calculating by using airborne vector magnetic data based on the vector relationship of geomagnetic field . The calculation uses the magnitude of the vectors , , and through a simple approach. To ensure that each magnitude has the same level, we normalize the magnitude of using the total-field magnetic data measured by the scalar magnetic sensor. The method is applied to the measured airborne vector magnetic data at the Qixin area of the East Tianshan Mountains in China. The results indicate that the calculated has high precision and can distinguish the approximation error of approximately 0.1 nT. We also analyze the characteristics of the approximation error that are caused by the effects of different total magnetization inclinations. These error characteristics are used to predict the total magnetization inclination of a 2D magnetic source based on the measured airborne vector magnetic data.