For the purpose to measure the water-film thickness of a sea water pump slipper pair, a measurement system based on a reflective modulated optical fiber displacement sensor was designed and implemented. A new optical fiber probe structure which matches the construction of sea water pump was designed, in which a three-circle coaxial fiber bundle with better compensation function was used to eliminate the influences caused by the intensity fluctuation of light source, the character of reflective surface, optical loss and the bending loss. The signal conditioning circuit consisting of a light source，a power supply，a photoelectric conversion circuit, a low pass filter and a dividing circuit was developed to adjust the signals. The experimental test for static property and dynamic property of the system was performed and the results demonstrate that the system offers a precision of micron-level and a good dynamic property. Its measuring range is 1 000 μm ,and sensitivity is 3.45 mV/μm. This results verify that the measurement system proposed in this paper meets the requirements of the measurement of water film thickness.