The soft X-ray spectrograph is an important instrument for plasma diagnostics. As the core optical element of spectrograph, holographic flat-field grating is fabricated by aspheric wave-front recording optics, so grooves on the surface are curve. The curve grooves of the grating would affect the spectral image properties, thus influencing spectral resolutions. In the design of recording optics, only the groove density distribution on the surface in meridian line should be guaranteed, so optimized recording optics is not unique. Thus gratings with different curvatures of grooves but with expected groove density distribution could be obtained. For holographic flat-field gratings used in a 0.8-6 nm region, we analyze the influences of different curve grooves on the spectral image by ray tracing, and find that the almost straight grooves which are obtained by means of cylinder mirror can obtain the better spectral images. The theoretical results show that the spectral resolutions of grating with almost straight grooves are obviously improved compared with curve grooves, the theoretical spectral resolutions increase from 626 to 953 at 3 nm and from 635 to 1222 at 5 nm, respectively.