A kind of M-Z interferometer is designed， which consists of 2×2 coupler and 3×3 coupler and is successfully applied to the demodulation of the designed fiber Bragg grating（FBG） voltage sensors. Compared with other electrical and optical fiber voltage sensors， FBG voltage sensor system with M-Z interferometer has simple structure and low price. Temperature and voltage may be separately measured with two different wavelength FBGs. The measurement results show that the linearity of the system is quite good from 50 V to 450 V， and the problem of temperature cross sensitive of FBG is eliminated. So the measurement results are more accurate， and the system can satisfy the measurement requirements of power system.