We use a varied line-spacing (VLS) grating to improve the performances of the constant-length Rowland-circle spherical grating monochromator. The translation distance of the mirror-grating tank in a scan of certain wavelength range will get much shorter. This will make the instrument easier to realize. Based on the principle of monochromator, we study how to optimize the optical system with a VLS grating in detail. The main imaging performances of the instrument are also evaluated.