The relaxor-based ferroelectric single crystals grown by vertical Bridgman method are cylindrical ingots with no natural crystallization face. The crystallographic orientations are required before cutting and processing. In this work, we proposed a simple method for determining crystallographic orientations of ferroelectric single crystal using powder X-ray diffractometer and X-ray orientation instrument. Firstly, the Miller indices of the strongest diffraction peak of arbitrarily cut plane are determined by a powder X-ray diffractometer. Secondly, the cut plane of crystal ingots is determined according to the Miller indices of the strongest diffraction peak by an X-ray orientation instrument. Thirdly, the intersection direction of the two crystallographic planes is obtained by X-ray orientation instrument. The three-dimensional orientations of ferroelectric single crystal are obtained based on the intersection direction and the crystallographic planes of the strongest diffraction peak. The application of this method to Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals demonstrates that it is a high accurate, high efficient, convenient, and waste-free method for crystallographic orientation. In addition, this method is also suitable for other crystals.