To realize the calibration of grating direction and alignment between mask and wafer, on the basis of theory of line grating used for photolithography alignment, a novel method for eliminating angular displacement using phase slope is proposed. Line grating masks and their alignment principle are also given. Before alignment, there exists angular displacement between mask alignment marks and wafer alignment masks. The nature of Moiré fringes and their relation with physical parameters of grating are mainly discussed, and the corresponding calculation formulas are obtained. According to Fourier method of frequency domain, relation between Moiré fringe frequency components and stripes is briefly analyzed. Using extracted one dimensional phase of Moiré fringes in ranks, inner relation between phase slope and angular displacement is obtained by data fitting, and calibration of stripe direction is realized. Simulation experimental results show that this method is simple and reliable, minimum angular displacement with less than 0.02° can be identified.