The influences of the analyzed crystal bandwidth and the angular divergence of focused beams were eliminated by the high-index lattice plane diffraction, and a method for energy bandwidth measurement of focused beams was proposed. In addition, a DuMond diagram was used to analyze the measurement process of energy bandwidth, and a detection system was built on the hard X-ray spectroscopy beamline at the Shanghai Synchrotron Radiation Facility. Furthermore, the different high-index lattice planes of crystals were employed for the energy bandwidth measurement of focused beams under the same energy and diffraction angle. Besides, in the context of 10 keV beam energy, the change of energy bandwidth during the bending process of the collimating mirror was measured by Si(555) and the optimal energy bandwidth measured in the non-dispersive configuration of Si(555) was 1.50 eV, with a difference of less than 10% from the calculated value (1.40 eV) by the Shadow tracker. The results demonstrate that the high-index lattice plane diffraction of crystals can be used for the high-precision energy bandwidth measurement of synchrotron radiation focused beams.