The winding part of power transformer is the key to ensure the smooth conversion and transmission of electric energy. With the increase of service life, the probability of winding fault is increasing, but the effective detection and diagnosis of winding fault is difficult to solve efficiently. The method of fault determination and location based on the distortion of three-dimensional leakage magnetic field in transformer windings shows great research potential. However, the traditional three-dimensional magnetic field sensor can not adapt to the harsh working environment, and it is difficult to be used in the measurement of three-dimensional magnetic leakage of transformers. Although the magneto-optical crystal magnetic field sensor can adapt to the high voltage and high electromagnetic environment near the transformer winding, the current research mainly focuses on one dimension. In addition, the measurement range of the existing 3D magneto-optical crystal magnetic field sensor can not meet the measurement of transformer magnetic leakage, so the realization of 3D magnetic leakage measurement is of great engineering value for timely and effective detection of magnetic leakage and accurate determination and positioning of winding faults.