Abstract: A spectrometer for high-harmonicsources is designed. The spectrometer uses an off-plane mounted plane grating to separate all harmonics. The separated diffracted beam is focused on the detector plane by a toroidal mirror.The total theoretical efficiency of the system exceeds 60%. The source of aberration and the compensation methods are analyzed for the toroidal mirror-plane grating composite system. The aberration introduced by grating diffraction can be compensated by optimizing the orientation angle of the tire mirror. In the extreme ultraviolet spectral range of 21.1~49.3 nm, the average spectral resolution λ/△λ is improved from 27 to 100, and the resolution at the central wavelength could reach 236. The spectrograph design can also be used for X-ray spectral separation and detection by switching to grating with higher groove density.