A fiber optic displacement sensor based on the principle of collimated beam reflective intensity modulation is constructed. Both emitting fiber and receiving fiber are connected to graded-index (GRIN) lenses. The probe GRIN lens emits a collimated beam to illuminate a sample at an oblique incident angle. The receiving GRIN lens is then placed in the direction of specular reflection. Modulation functions are analysed for specularly reflective and Lambertian diffusive samples. Artificial calculation shows that the sensor has high signal level and flexibility in working distance and measurement range through adjusting the configuration parameters.