Backscattering coupling effect in square ring resonator is studied. Vector method has been proposed to analyze the optical path variation induced by reflectors′ displacement in square ring resonator. A mathematic model of backscattering coupling analysis has been established. This model can be applied to analyze backscattering coupling effect in any form of ring resonators. Backscattering coupling coefficient r as a function of mirror′s axial displacements has been obtained. The absolute value of r has a period of two wave lengths in optical path length. It shows that the initial machining error of the resonator has a great effect on backscattering coupling effect. It is important for the research on the backscattering coupling effect in ring resonator.