Mostly, the principle of FGB strain sensor is to detect the shift of the central wavelength of FBGs. Now, we propose a new strain sensor using light speed extraordinary controlled method in a periodic structure modulated optical fiber. According to light speed which can be modulated in a photorefractive material in which written volume-index grating by two laser beams in opposite direction of fiber, the light signal may transmit with phase modulating due to intervening of another frequency shifting laser beam. In the experiment, the inner grating would be written in an optical fiber in which suitable little Stannum has been doped. Based on quantum theory, the energy-band structure has a zero slope at the edges of Brillouin zone and the group velocity of photons at the band structure of a periodic structure is zero. Due to the effect of stress, optical principal axle rotation angle of each beam is shifted, inducing the inner effective grating changed. During the testing period, pressure is put to a point of the fiber and fiber would be partly in uniform magnetic field. Under this environment, according to elasto-optical effect and magneto-optical effect, the pressing position and pressure value can be determined. This research work is supported by the National Natural Science Foundation of China under grant No.60472023.