A high-energy X-ray microfocal spot has an important application in high-energy microbeam X-ray analysis. To obtain a high-energy X-ray lens with a microfocal spot, a tapered monocapillary lens was designed in theory. This lens can focus X-ray with an energy of 100 keV onto a micron-scale focal spot with a gain in the focal spot diameter and power density of the order of microns and tens, respectively, and the corresponding focal length is ~5 mm. Furthermore, the characteristics of the lens with a glass and a gold reflecting surface were simulated in theory, respectively. The flux in the focal spot of this X-ray lens with a gold reflecting surface is 14.8 times that with a borosilicate glass surface. The simulation results are helpful in the fabrication and application of such tapered monocapillary X-ray lens.