In the field of reflective fiber-optic displacement sensor, in order to improve the displacement measuring accuracy, two receiving fibers are often used for intensity and reflectivity compensation. Based on the mathematic model of light coupling between the fiber pair, three types of tri-fiber optic sensors are studied. The general mathematic model of the tri-fiber sensors is proposed. Simulation about them is finished. Research shows that these kinds of tri-fiber sensors can not only improve the linear range, linearity and displacement measuring sensitivity, but also eliminate the influence caused by power fluctuation of the light source and variety of surface reflectivity.