Abstract: The pressure sensing mechanism of fiber Bragg grating has been researched. Various physical effects that induce the center wavelength of the fiber Bragg grating to shift are discussed and the sensitivity coefficients of different physical effects under different types of pressure are calculated. The calculation results show that the pressure sensing coefficients under the axial and the transverse pressures are opposite in sign, and the pressure sensitivity can be increased through shielding either of them. The strain sensitivity induced by the waveguide effect is several orders of magnitude smaller than that induced by the strainoptic effect, so it can be neglected. The conclusion that the strainoptic effect has different contributes to the stress sensitivity under pressure in different directions is also drawn for the first time.