Substrate positioning error in exposure system will directly harm the groove accuracy of holographic grating. It will cause grating period error in the plane holographic grating, or increase the aberration of the concave holographic grating. The effects on grating period with the positioning error of the standard grating as well as the substrate are analyzed, while moiré fringe method is used to adjust the interference field period in the Lloyd′s mirror interference system of holographic grating. The positioning error which can be compensated by adjusting the substrate parallel to the standard grating surface is pointed out.