The invention relates to a sapphire optical fiber probe sensor used for measuring gas holdup under an oil well, which comprises a probe, an optical fiber coupler, a light source and a light detector, the probe is a sapphire sensitive probe, the optical fiber coupler is a Y-shaped optical fiber coupler, the light source is infrared source, the sapphire sensitive probe and the Y-shaped optical fiber coupler are sealed in a stainless steel protective casing, and the infrared source and the light detector are sealed in a stainless steel sealed chamber connected to the stainless steel protective casing. The sapphire optical fiber probe sensor used for measuring gas holdup under the oil well has the advantages of high sensitivity, less optical loss, high measurement accuracy, reliable performance, electromagnetic interference resistance, corrosion resistance, oil contamination resistance, high pressure resistance, high temperature resistance, good electrical insulating property, safety explosion resistance, long usage life, exquisite structure, small volume, light weight and less power consumption, and the sapphire optical fiber probe sensor enables real-time and on-line measurement for gas holdup of oil-air-water three-phase flows in a narrow space under the oil well.