Milling straight edges is currently a vital standard gauge for the verification items of flatness. Two methods for measuring milling straight edges based on phase shifting interferometer are put forward, whose advantages are non-contact, high sampling density and high accuracy frequency measurement. One is the sub-aperture stitching method; the other is the oblique incidence method. The simplification in verification items was achieved by using standard optical flat directly and long optical flat was excluded in the items. Take Φ150mm phase shifting interferometer and 300mm milling straight edge as an example, this paper introduces the principles and measuring results of the two methods, and gets flatness (sparse points) from the data of phase shifting interferometer to keep to the requirements of the verification regulation. Finally, the author summaries and estimates the uncertainty of measurement in contrast with the verification regulation. The normalized error of the two methods is less than 0.5.