A new reflective fiber optic displacement sensor with multi-concentric distribution bundles is presented, whose mathematical model are set up and numerically simulated. The relations between the output characteristics and the various parameters have been discussed. The mathematical model indicates that the ratio of two outputs of the fiber bundle can eliminate the influence on the measured result caused by the unsteadiness of incident light intensity and the change of surface reflectivity. It is of advantage to enhance the ability of anti-interference and the precision of measurement. The simulation results resemble the earlier experimental results, and are suitable to optimize the structural design of this kind of sensor.