We report a high-efficiency hard-X-ray resonator with inclined-incidence geometry. A beam incident at 36.87° with respect to [3 1 0] excites Bragg back diffraction along (12 4 0) at 14.4388 keV for resonance in a Si-based resonator to produce intense resonance fringes. The experimental results showed the visibility enhanced by nearly 30 times compared with normal incidence. Also numerical calculations of the inclined-incidence resonator demonstrate ultrahigh efficiency and extremely narrow resolving power (sub-meV) with low background. This geometry surpasses the intrinsic limits of normal-incidence crystal-based resonators and enables ultrahigh-resolution X-ray optics for X-ray diffraction, spectroscopy, and imaging applications.