Metrological gratings are now widely used in such fields as precision measurement, numerical control machine and so on. The mutual position change between instruction grating and scale grating of the sensor during grating installation or application procedure will result in the distortion of Moiré fringe, which directly influence the measurement accuracy and complexity degree for following process system. In this paper, Fourier spectrum analysis method was applied to build the optical field output model involving instruction grating and scale grating pose parameters, to introduce the mathematic formulas for output optical field with the two gratings unparallel mutually and to detailedly analyze the main influencing factors, all of which are simulated in Matlab. Moreover, this paper gave Moiré fringe error values and their variation trend for different assembling offset angles, discussed measuring results affected by configuration dimension of receiving sensor and obtained that the receiving system with its dimension the same as Moiré fringe had the minimum measuring error. The results are of great guiding for accuracy and configuration design of grating couples.