System and method for measuring grating constant

The invention discloses a system and a method for measuring a grating constant. The system comprises a light source, a beam expanding collimation system, a moire fringe imaging system and an image acquisition system, wherein probe light emitted by the light source passes through the beam expanding collimation system, and is imaged in the moire fringe imaging system to obtain moire fringes; and themoire fringes are acquired and converted into a fringe-shaped chart with width and interval the same as that of the moire fringes by means of the image acquisition system. The method based on the system comprises the steps that the probe light emitted by the light source becomes a beam of parallel light after passing through the beam expanding collimation system, the parallel light enters the moire fringe imaging system comprising a standard grating and a grating to be measured to form the moire fringes, the moire fringes are acquired and converted into the fringe-shaped chart with the widthand interval the same as t