A new plane grating monochromator is used in the surface-physics beamline at the Hefei National Synchrotron Radiation Lab. (NSRL) as its light-dispersing instrument. Its optical imaging system consists of a plane grating and a spherical mirror. The wavelength tuning is performed by rotating the pre-spherical mirror around an axis outside of the surface while rotating the grating around its center. The monochromator offers high optical transmission and high spectral resolving power at 4～120 nm wavelength range.