The Letter reports the thermal stability and strain response of Fabry–Perot (FP) cavity under different high temperatures. The FP cavity was made by thermal regeneration of two identical cascaded fiber Bragg gratings (FBGs). It is demonstrated that the FP cavity is capable of measuring temperatures from 300°C to 900°C with a temperature sensitivity of 15.97 pm/°C. The elongation of the fiber was observed through the drifted Bragg wavelength at 700°C or above when weight was loaded. The elongation was further inferred by the slight change in the interference spectra of the FP cavity at 900°C.