In this paper，the formation of the optical beat in double grating diffraction and the related principle of weak vibration measurement are presented. By comparing the simulation results with the experimental results，a modification of the optical signal expression is established. It is found that the vibration of the grating induces the vibration of the diffraction spots and causes the modification. In addition，the mechanism of the burr signal caused by the senior diffraction of the grating is revealed and the suppression methods are studied. One of the methods is to make two gratings as parallel as possible，while the other method is to suppress senior diffractions by reducing the intensity of light source properly.