Abstract: Diffraction gratings can separate the light of different wavelengths spatially，which have graduallyreplaced prisms for application in most areas of spectral analysis.Measuring the grating constant by grating diffraction is a typical experiment in the experiment of college physics.This paper introduces two interesting phenomena found during the grating diffraction experiment：(i)there is a minimum diffraction angle when rotating the grating angle，and (ii)when the grating is rotated to a certain angle，the diffracted spot will gradually disappear into the field of view.Then，we analyze and explain these two phenomena in theory，and we design two methods for measuring grating constants.Moreover，we verify these two methods in experiment.Compared with the traditional method of measuring the grating constant with the minimum deflection angle，this paper analyzes the influence of the position deviation of the minimum deflection angle，and proposes a method of symmetric rotation measurement to further reduce the error.

Key words: grating diffraction, grating constant, diffraction angle