In the manufacturing industry, the flatness error of some components has reached the order of tens of micrometers, exceeding the measurement range of traditional interferometers. To solve this problem, a large angle grazing incidence interferometry measurement method can be used, such as the prism method with variable inclination angle and small gap phase shift interferometry. To solve the problem of high surface reflectivity under large angle grazing incidence, which forms multi beam interference and increases the difficulty of phase extraction, two methods are proposed: reducing multi beam interference through reflectivity control film system and eliminating multi beam interference through dynamic film system. By analyzing the system using admittance matrix and simulating the incident angle of 83 ° using Matlab, when the reflectivity of the right angled prism chord is controlled at 10%, the error in phase shift calculation caused by multi beam interference will be reduced to around 7.2nm. Finally, the dynamic film design and actual equipment that can completely eliminate multi beam interference were introduced.