In lithographic alignment, alignment labels located on the mask and wafer are generally made by two gratings of slightly different periods. The relative position between the mask and wafer is detected by phase information of moiré fringes. In practical application, the direction of moiré fringes is not only related to geometric position of alignment labels, but also position of CCD. To propel the method of lithographic alignment into practice, phase distribution law of general gratings is analyzed from rectangle gratings to moiré fringes. Based on phase characteristics of moiré fringes, effects of geometric positions of the template, wafer and CCD on alignment precision are emphatically analyzed. Mathematical-relation models between the alignment deviation and theoretical displacement are also established. Research results show that the maximal alignment error is theoretically less than 0.002 pixel without an angular displacement when the displacement is less than 0.4 pixel.