A low cost curvature optical fiber sensor that can detect curvature directly is developed in recent years; its curvature measurement sensitivity is improved using bend enhanced method and the sensor can distinguish between positive bending and negative bending. This sensor has a wide linear range and is suitable for measuring large curvature deformation. It meets the demand for embedded measuring in structure. With transfering strain to curvature, it can also measure axial strain. Nevertheless, the operation principle of this sensor is still under exploration. Through analyzing radiation law of cosines,whisper gallery ray and groove angle theories, the operation principle of this intensity modulated macro-bend curvature optical fiber senor is presented based on light scattering theory: bending of sensitive zone bring about mode coupling and lead to the variation of surface scattering loss. The mathematic model of relationship among light loss, bending curvature, surface roughness and parameters of the fiber’s configuration is also presented. Experimental results have demonstrated the proposed model.