Abstract: This paper presents the principle, structure and aberration-corrected designing of a flat focal field concave grating spectrograph. We designed and constructed a spectrograph instrument using varied spaced concave grating with 2 400 grooves/mm. Spectral range of detection is 1.5～10nm. Typical soft X-ray line spectra emitted from laser plasma of C5H16O6 target has been obtained with this spectrograph instrument on 'Xing-guang II' laser facility. The experiment results indicate that the spectral resolution is 0.02nm. Absorption spectrum of laser-produced plasma has been observed successfully with the device.