The principle of all-optical switching in fiber Bragg grating (FBG) is illustrated. Yb-doped phase-shifted FBG is adopted to decrease the need for the power of pump from the aspects of structure and material. The transmittivity of phase-shifted FBG is computed using the method of transfer matrix. The result shows the availability of decreasing the power of pump by using the phase-shifted FBG. The extinction ratios are computed under different pump powers. Theoretically, low power all-optical switching in phase-shifted FBG is promising.