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 was designed to solve the above problems. Sapphire was used as the front sensing material of the optical fiber probe. Due to the difference in refractive index of light between the gas phase and the liquid phase, the light intensity of the reflected light from the probe was detected to distinguish whether the optical fiber probe was in the gas phase or the liquid phase. 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, the measurement of the three-phase phase holdup of oil, water and gas in the oil well can be realized with a single probe sensor.