A stabilizing technique has been developed for a 3×3-coupler-based Mach-Zehnder interferometer, and the interferometer is used to interrogate a fiber Bragg grating (FBG) sensor. The fluctuation of an output can be removed by using the technique. New arithmetic is also introduced to recover the wavelength shifts of the FBG. The experimental results show that stable wavelength shifts of the FBG were obtained. The accurate calculation of modulation amplitude was also demonstrated.