A Talbot interferometer used for fabrication of fiber Bragg grating is analyzed in this paper. As the light ray tracing method can not give the actual intensity distribution of interference field when spatial coherence and temporal coherence of excimer laser are taken into account, electromagnetic theory is used for the detail description of the near interference field and far interference field, and a computation model is established, which considering the turning angle of mirrors as a model parameter for the first time. With the model, a series of simulations are carried out. From the results of calculations, we obtained a more actual status of interference field. The results showed that spatial coherence had a large influence on fringe visibility of both near and far field interference, and the far field was stricter with spatial coherence. The effect of turning angles of mirrors on far field interference cannot be negligible