The theoretical model of the eccentric core fiber is established according to the traditional optical transmission theory.Moreover, the expression of evanescent wave of eccentric core is deduced according to Fresnel equation so that the depth of penetration is solved.Through the structure modeling of eccentric core optical fiber completed by BeamPROP module, the variation trend of modal field distribution, evanescent wave, as well as fundamental mode effective index with the eccentric distance is obtained.The simulation result shows that the evanescent wave declines with the increase of δ.The fundamental mode effective index increases as δ dose, and it will be stabilized when δ equals 8 μm.