The alignment method based on moire imaging has been wildly used for its high accuracy of physical measurement, which utilized the phase of moire fringe to measure the relative linear displacement between the mask and the wafer. However, this method is only theoretically accurate due to the affection of certain parameters, such as the optical beam collimation. In this paper, the influence of collimation on the alignment accuracy is thoroughly analyzed. The theoretical analyses and simulation results indicate that the alignment accuracy, which was observed just behind the test grating, is sensitive to the divergence or convergence angle of incident light. On this basis, the method for the error correction is proposed and confirmed.