In this paper, the results of the long-term temperature monitoring of the compact muon solenoid experiment (CMS) at CERN are shown. The measurements were carried out by means of a system based on fiber Bragg grating (FBG) sensors in wavelength-division multiplexing (WDM). Due to the harsh working conditions at the CMS, the FBG sensor represents the ideal candidate to realize a reliable and accurate sensing system. The sensing principles of the FBG sensor and its temperature characteristics are introduced. A temperature monitoring system based on FBG for high-energy physics applications is designed and installed. The sensing system was used successfully last year in monitoring the temperature of CMS bulkhead. The reported results show good reliability and high accuracy of the FBG sensing system during the long-time working stage.