Using the wave coupling theory of the linear electro-optic effect,a new method to measure mini rotating-angle is developed,which is with high precision. According to the wave coupling theory of linear electrooptic effect,advanced by W. L. She et al.. The relation between the output intensity and the incident angle is formulated. That is to say,the displacement of the incident angle could be determined accurately by measuring the output intensity. Based on this principle,a simple system was presented to measure the small rotating-angle of an object,meanwhile,its measurement range and accuracy could be modulated by changing the external electric field or the wave length of incidence. Its measuring accuracy can be 3.5 within the measurement range of 3′ when the electro-optic crystal is LiNbO₃.