The mature three-phase flow phase holdup measurement sensor on the ground cannot adapt to the complex environment of high-temperature and high-pressure corrosion, and the existing downhole measuring equipment is complex. A fiber optic probe sensor is designed to solve the above problems. Sapphire is used as the front sensing material of the optical fiber probe. The optical fiber probe is in gas phase or liquid phase by detecting the different refractive index of the gas phase and the liquid phase to the light, and the different electrical signals are generated by detecting the changeover the light intensity reflected back to the receiver by the optical fiber probe. By detecting the fluorescent light intensity coupled to the optical fiber through the optical fiber probe to distinguish the water phase or the oil phase, so as to realize the measurement of the three-phase phase holdup of oil, water and gas in the oil well with a single probe sensor.