Slit curvature becomes an important factor which introduces the decline of spectral resolution in the developing of spectrograph with high NA. Slit curvature is corrected according to the correction formula obtained from the grating equation of off- axis ray. To cope with the problem that the real image of spectral lines usually deviate from the formula at ideal condition, calibration with a spectral lamp which has lines distributed across the full span of the spectrograph is used to fit out the assembly error parameters. Neon lamp is used to fit out these parameters, slit curvature in the spectral image of krypton lamp and fluorescent lamp of mercury vapor is corrected with these parameters. Resolution has be significantly improved, which is nearly close to the spectral resolution limit of the spectrograph when irradiated by a spot light source.