A method for testing the 90-deg error of dihedral angles of glass cubes is proposed. The test beam of a Fizeau interferometer enters the cube and is retroreflected by a dihedral angle to interfere with the reference beam. The angle between different regions of the retroreflective wavefront can be obtained using a phase-shifting interferometer. The relation between subwavefronts and the dihedral angle error is deduced, so the latter can be calculated. It does not need any extra auxiliary optical components. In the experiment, a glass cube is tested on the phase-shifting interferometer, and its four dihedral angle errors in the same principal section are obtained. The results obey the angle sum of a quadrangle principle with a tolerance of 0.04″. Moreover, the result is validated by another method, as the differences between them are less than 0.06″.