The physical model for the inscription of mid-infrared Fiber Bragg Grating (FBG) is established based on the Finite Element Method (FEM). The diffracted light field by the phase mask is analyzed. Two light sources as the plane wave and spherical wave are adopted in the simulation. When using the plane wave as the incident beam, the variation of the Talbot diffraction pattern is deeply analyzed by changing the incident angle of the plane wave and the groove depth of the phase mask. When using the spherical wave as the incident light, the variation of the diffraction pattern of the phase mask is analyzed by changing the distance between the light source and the phase mask. As a result, when the plane wave is incident in an arbitrary angle, the Talbot diffraction pattern is formed along the same direction as the input. The profile and maximum energy of the Talbot diffraction is changed with the groove depth of the phase mask. With the groove depth increasing, the maximum diffraction energy increases gradually. However, it begins to decrease after the depth reaches certain value. At the same time, the Talbot image tends to evolve to the uniform fringes. When the spherical wave is incident on the phase mask, the distribution of Talbot diffraction pattern is along the propagation direction of the spherical wave. As the distance between the spherical wave and phase mask is decreased, the maximum diffraction energy is higher. Then it begins to decrease after the distance is smaller than some certain value.