An ultra-sensitive fiber optic sensor based on a HO(Me2SiO)nH (silicone rubber) Fabry-Pérot (FP) resonant cavity is fabricated through a simple method of dipping, which has high linearity, high sensitivity, and a wide response range to temperature. The silicone rubber can form a smooth sphere without wire drawing, and it has strong heat resistance and aging resistance. Its favorable waterproof performance enables it to work efficiently in different humid environments. A method of synchronizing the detection and wavelength sweeping signal is applied to analyze the wavelength shifts of the interference signal. The sensitivity of the fabricated FP sensor is almost 400 pm/°C in the range of 50-190°C with a linearity of 0.999, and the resolution is 0.002 nm. In addition, the testing results at room temperature can also satisfy the linear relationship, providing the possibility of applications in the biological field.