The reflective itensity-modulated optical fiber sensor for measuring displacement, the antenna-shaped optical fiber sensor, is usually composed of a fiber bundle with the emitting and receiving fibers in a specific arrangement at the probe tip. The three elementary types of fiber arrangement are: concentric distribution, semicircular distribution and random distribution. In comparison with the size of the optical fiber sensor, the thickness of clad and binding element of the fibers is so small that it can be ignored. The mathematical models of the modulation function for the single fiber couple composed of both an emitting fiber and a receiving one are presented. After analyzing the distribution of the fibers inside the optic fiber sensor by using the geometric and statistical theories, the general mathematical models of modulation function for the three types of the sensors mentioned above are proposed. And all the models are simulated with computer and all the simulating results are briefly analyzed.