An electrically tunable fiber Bragg grating based on thermally poled twin-hole fiber is designed. The formation of frozen-in electric field during thermal poling is analyzed based on the two-dimensional charge dynamics model. A periodically poled Bragg fiber grating is designed based on the role of ultraviolet light in the second-order nonlinearity (SON) periodically erasing of thermally poled fiber and the fiber grating manufacturing. The tunability is achieved as the variation responses of refractive indices to modulating voltage in SON erased zone and SON zone are different. The influence of the initial refractive index and the length of the grating on the tunability are also investigated. The electrically tunable thermally poled fiber Bragg grating has important reference value for the application of fiber Bragg grating and thermally poled optical fiber in active optical devices.