In order to realize the purpose of measuring angle quickly and accurately in the process of cube machining， and to solve the problem that traditional angle measuring technology cannot adapt the mass processing of cube， the method of measuring cube right angle by plane interferometer is studied in this paper. The relationship between the angle deviation to be measured and the number of interference stripes is established by theoretical derivation. At the same time， it provides the guidance of theory and detection method for the correction of on-line machining process angle. Through the 90 mm×90 mm×90 mm cube to explore the angle measurement process， the results show that when the number of fringes in the filed of view m≤5.5， the right angle error δ≤2″. When the retardation increases， it is a positive error when the fringe shrinks， while the fringe diffusion is a negative error. The measurement of cube right angle error by plane interferometer and the judgment of error positive and negative are realized.