As one of the important technical specifications of holographic gratings, the diffraction wavefront aberration directly influences the grating resolution, which depends on the light path adjustment. The q-parameters of Gaussian beam are used to describe the propagation and the transformation by the collimating system of the exposal system; the spatial phase of the two Gaussian beams and the mathematical expression of Moiré pattern are given; then the relation between the system adjustment error and the wavefront aberration of the resulting interference image is discussed. The conclusion shows that the relative defocus of the left and the right collimating systems is the most critical parameter in exposal system, which influences the wavefront aberration strongly. The wavefront aberration also increases according to the decreased focal length. The results obtained from the initial set of experiments are in good agreement with th