The spectral resolution of the spectrometer will be seriously deteriorated by radius error which is inevitable in the flat-field holographic concave grating fabrication. In order to give theoretical guidance to the design and assembly of the spectrometers, the geometric ray-tracing method was used to calculate and analyze the variation of the spectral image width with different radius error. It was discovered that in a wide range of the radius error, the position of the tangential focal curve moves forward or backward as the radius changes, while the shape of the tangential focal curve remains almost unchanged. As is shown through the calculation, the radius error can be compensated by adjustment of the detector's position or entrance slit distance. The simulation results indicate that with proper modification of the using structure, the grating can have a spectral resolution as good as the designed results.