In order to improve performance and reduce the insertion losses of electro-optic switches, a fiber Bragg grating(FBG) optical switch was designed based on a dual-pass tunable Mach-Zehnder interferometer. The filtering cycle spectrum was tunable by changing the delay time of the electric adjustable fiber delay line in the interferometer. The filter principle of the optical fiber M-Z interferometer was analyzed and tested with the C-band broadband light, and the results show that it can achieve a wide range and high-precision filter adjustment function. The interferometer was also tested as a FBG optical switch. The output spectrum at a wavelength of 1550 nm was obtained through a FBG optical switch performance testing system experiment. Its side mode suppression ratio was over 25 dB. Test results show that it can achieve a large range and fine filter, and that the optical switch also has advantages of a high extinction ratio, simple structure, and easy adjustability.