The characters of ring interferometer as a white-light shearing interferometer have been discussed. The simplest type of ring interferometer is constructed by threa plates. This interferometer can have definite lateral shearing with equal optical path. The adjustment methods are discussed. Ring interferometers formed with four and/or even more plates are analysed, it has been shown that, ring spacial optical path with four plates can form a rotational shearing interferometer with arbitrary shearing angle. The behavior of optical system in ring interferometer is discussed. It ahows that, the ring interferometer with definite focal length or telescopic system can obtain the radical shearing expected. Finally, the tolerance and measurment accuracy of ring shearing interferometer are presented. This paper waa completed in 1976. Following the idea proposed in this article some interesting experiments were performed, aach as a paper published in IEEE J. of Quantum Electronics, QE-17 (1981) 1768.