Abstract: A method for solving the ultra-fine-group slowingdown equation based on the method of characteristic was proposed to realize the rigorous resonance calculation of the lattices with complicated geometries and neutron spectra. By coupling the fixed source calculation of the method of characteristic, the ultra-fine-group slowing-down equation was established within the resonance energy range, and the resonance self-shielding cross sections were obtained through the ultra-fine-group neutron spectrum. The problems including the typical PWR pin-cell problem, the pin-cell problem with temperature distribution, the pin-cell problem with intra-pellet fuel division and slab fuel assembly problem were performed. The results show that this method can obtain high accuracy on complicated geometry and complicated neutron spectrum problems, providing a reference for resonance self-shielding calculation.