With equivalent conductance method,the geometry optics theory of step fiber,the light propagation characteristics of Bragg fiber structures have been researched in this paper.First,the thickness and layer number,the reflectivity of which reaches 99.99%,have been designed;Second,the propagation effect which follows the change of wavelength,layer number and the angle of incidence have been analyzed.Third,it have been concluded when the NA of beam of light is less than 0.15 (the incident angle is 81.37°),the reflectivity reaches 99.99%.This research possesses an important meaning for the manufacture of Bragg fiber and the coupling of step fibre.