A new laser displacement measurement interferometer with nanometric accuracy has been developed. A new method called Coupled Differential Interferometry is proposed. The resolution and the stability of the interferometer are improved by multiplicating the optical path difference (OPD). The new interferometer is simple in concept, symmetric in optical paths, without optical deadpath, easy to set up and align. A λ/1600 resolution and a few nanometer accuracy are achieved within the range of 10 mm.