The ultraviolet interference field behind the phase mask for fabricating fiber Bragg grating (FBG) and the interference contrast are studied. The relationship between the interference contrast of light field and the refractive index modulation of FBG is taken into account in this paper. The experimental results show that increasing the distance between the optical fiber and the phase mask makes the interference contrast decrease. The reduction of interference contrast leads to an increase of loss in FBG. The numerical investigations also demonstrate that the increase of non ±1 order diffraction energy, the deterioration of the time and the spatial coherence of ultraviolet (UV) laser and the increase of oblique incident angle of UV light beam could reduce the interference field contrast. In order to fabricate low-loss fiber gratings, these unfavorable factors must be overcome. The results provide the technical basis for the selection of ultraviolet laser source, phase mask and optical path alignment in the FBG fabrication used in high power fiber lasers.