Soft X-ray spectrographs based on flat-field diffraction gratings play important roles in the fields of laser plasma diagnostics, etc. The flat-field gratings fabricated by electron beam lithography-near field holography (EBL-NFH) integrate the advantages of both sides, including the flexibility and high accuracy of spatial distribution of groove density due to EBL, and low stray light and low high-order harmonics similar to those of holographic gratings. This paper simulates the effect of dominating fabrication errors during EBL and NFH on the spectral images of flat-field gratings based on ray tracing theory. The results indicate that the fabrication errors of EBL and NFH will broaden spectral lines and degrade spectral images. We take a typical soft X-ray flat-field grating with a central density of 2400 lines/mm ranging from 0.8 nm to 6.0 nm as an example, when the length of the fused silica mask is 50 mm (along the direction of grating vector), the segment number should not be less than 1500. And the broadening of spectral lines due to the groove density error of the fused silica mask during EBL can be compensated by adjusting the parameters of NFH, such as the spacing and angle between the fused silica mask and the photoresist-coated grating substrate. The dominating factors are spacing and angle between the fused silica mask and the photoresist-coated grating substrate. And during NFH, rather than removing every fabrication parameter errors, the spectral line broadening can be eliminated by optimizing relative values among different errors. The simulation results are helpful to optimize the EBL-writing strategy for the mask, reduce the complexity of mask fabrication, and design and adjust the light path of NFH.