A large‐aperture high‐accuracy phase‐shifting digital flat interferometer that is a combination of optics, mechanics, electricity, and algorithm is described. The aperture size is 250 mm and a liquid surface is used as an absolute flat to eliminate system error. The accuracy is better than λ/50 (λ=0.6328 μm, peak‐to‐valley value). The tested aperture can be enlarged to 500 mm. This interferometer has been used as an optical flat standard instrument for China. The optical interferometer, phase shifter, and calibration of precision are described. © 1996 Society of Photo−Optical Instrumentation Engineers.