The stability of interference fringes in a holographic exposure system directly affects the quality of grating masks. The characteristics of the reference interference fringes are investigated and the detection methods of phase, tilt, and period of the grating are obtained. A three-dimensional (3D) locking system is designed in order to improve the stability of this grating exposure system. The reference interference fringes are detected by a camera. The optical path is finely adjusted by the micro-motors and piezoelectric ceramics. The phase, tilt, and period of the grating are effectively controlled. The experimental results show that the 3D fringe locking system improves the stability of interference fringes, shortens the setup time before exposure, and further enhances the grating contrast, which is benificial for improving the quality of a large-aperture holographic grating.