This paper proposes a method that combines etching, masking, and magnetron sputtering techniques to prepare a ring-shaped silver mirror structure on the end face of a standard multimode optical fiber. By utilizing the capillary effect to fill a capillary glass tube with polydimethylsiloxane (PDMS), a structure consisting of a PDMS cavity and an air cavity is made, resulting in the achievement of a fiber optic dual Fabry–Perot (F–P) cavity temperature sensor with a ring-shaped silver mirror structure. When the external temperature changes, both the cavity length and the refractive index of PDMS change, causing variations in the intensity of interference light. The annular silver mirror utilizes its high reflectivity to allow more light to enter the receiving end, resulting in a more pronounced change in photon count. Within the range of 25–80 °C, sensitivity of 150.74 cps/°C and linearity of 0.998 have been achieved. Our optical fiber temperature sensor has demonstrated cost-effectiveness, wide range, and high stability in temperature detection through multiple repeated experiments.