This paper presents a method of measuring neutral point DC component of transformer using AC transformer. First, the method of saturation recognition based on correlation coefficients is used to judge whether the AC transformer is saturated or not. The third harmonics content is measured by the neutral point AC transformer. Through the relation curve (saturated curve or unsaturated curve) between the third harmonics content and neutral point DC component, the DC components corresponding to the three harmonic content can be obtained. Comparing the size of DC component, the expansion of the accident is prevented by alarming or tripping. This method does not need to add an extra equipment, and it is simple and worthy of promotion.