Concave grating has both imaging and dispersion capabilities, monochromators using concave gratings are well miniaturised and can be applied to the vacuum UV band below 200 nm. The spectral resolution and wavelength repeatability are important indicators of monochromators, for the spectral resolution of monochromators, this paper combines the intrinsic resolution of the grating and the broadening caused by the slit to derive a model for calculating the spectral resolution of monochromators. Experimental verification using our own micromotion slit, the monochromator resolution is in accordance with the theoretical model, with a limit resolution better than 0. 1 nm. For the wavelength repeatability of monochromator, the wavelength repeatability influencing factors are analyzed on the basis of the conversion of monochromator optical machine structure parameters, the wavelength repeatability calculation model of monochromator is derived, and the wavelength repeatability using mercury lamp as the light source is verified to be better than 0. 02 nm in accordance with the theoretical calculation model, which verify the validity of the structural design of the optical machine and the correctness of the theoretical analysis.