This paper describes a large aperture-high accuracy phase shifting digital flat interferometer which is a combination of optics,mechanics,electricity and algorithm.Its aperture is 245 mm and liquid surface is used as absolute flat to eliminate systemic 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 will be used as an optical flat standard instrument.The optical interfermeter,phase shifter and calibration of precision are discussed.