A high-sensitivity and miniature open cavity Fabry–Perot interferometer (OCFPI) encapsulated with the polydimethylsiloxane (PDMS) film based on high-order harmonic Vernier effect is designed and experimentally investigated. To the best of our knowledge, PDMS is applied for the first time to fill the open cavity of Fabry–Perot interferometer to obtain high-temperature sensitivity. The resonant dip (peak) wavelength of the designed temperature sensor monotonically moves toward the shortwave direction as the temperature increases from 40°C to 60°C due to the effects of expansion and thermo–optic property of PDMS. The proposed OCFPI encapsulated with PDMS film provides the following excellent performance advantages. (1) Compared with traditional all-fiber air-cavity OCFPIs with temperature sensitivity of approximately 10 pm/°C, the proposed OCFPI sensor has a much higher temperature sensitivity of -3.4 nm/°C at the temperature range of 40°C–60°C with a magnification factor ( M -factor) of approximately 11 when order of harmonic Vernier effect i = 4. (2) The proposed OCFPI exhibits good reversibility during the heating and cooling processes, and the measured M -factor matches well with the theoretically calculated M -factor. (3) The proposed OCFPI shows excellent stability with maximum wavelength deviation of 0.567 nm (internal envelope based on a fourth-order harmonic Vernier effect) and 0.042 nm (upper envelope) within 450 min. (4) The proposed OCFPI is inexpensive, robust, easy to fabricate, and compact, which can be used in harsh environments. Therefore, it provides excellent potential in dynamic temperature measurement.