An accurate modeling method for the internal structure of hollow- core photonic bandgap fiber is proposed, and the effect of core structure on scattering loss is studied by full-vector finite element method. The fibers with different core walls thicknesses and different core radii are simulated, and the normalized interface field intensity is used to characterize the scattering loss. Calculation results show that scattering loss can be largely reduced with the relative core wall thickness Tc≈4. Furthermore, scattering loss decreases with the increase of core radius. With optimized core structure design, the scattering loss can be further reduced down to 50% of the present level in theory within the wavelength range from 1.5 μm to 1.56 μm.