A high-reliability optical fiber probe was designed and developed for the void fraction measurement of the oil-gas-water three-phase flow in oil and gas wells, for which, a conical sapphire structure was used as the sensitive tip of the probe. The gold-based solder was exploited in order to seal the sensitive tip and the transition tube, as well as the transition tube and the protective tube. Experiments were carried out to test the reliability and void fraction, and the results show that the designed optical fiber probe is able to withstand high temperature (150℃) and high pressure (100MPa) with a high reliability. In the pipe string, the void fraction of the three-phase slug flow is 46%. The research results not only verify the feasibility of using optical fiber probes to measure void fraction of oil and gas wells, but also provide theoretical basis and practical guidance for the follow-up engineering application.