Wavelength of Fiber Bragg Grating(FBG) changes with ambient temperature and pressure. If change of wavelength is detected, then ambient temperature and pressure can be calculated. In this paper, a highly stable Fiber Bragg Grating(FBG) wavelength demodulation system based on F-P etalon with tempe-rature control module was proposed. The theory of the FBG wavelength demodulation system was discussed and the reason of high stability was analyzed. In this system, a Fabry Perot(F-P) etalon with temperature control module was introduced for real-time wavelength calibration. Wavelengths of F-P etalon can be inquired by the specification. Wavelength of FBG can be calculated by linear interpolation. The temperature control module can ensure temperature of the etalon changes within ±0.01 ℃, so that wavelengths of the etalon can be regarded as constant values. In the end the stability of the system was experimented by measuring wavelengths of FBG which was in a water-bath. The system was also compared with the MOI corporation optical sensing interrogator sm125 on the stability. Experimental results demonstrate that the system can achieve a long-term stability of ±0.15 pm in 20 h, while sm125 was ±3 pm.