Based on the theory about the design of spectrometer and error analysis, a method about error analysis for system alignment is proposed. By using Zemax software, the alignment error on the image quality is analyzed. The result indicates that the tilt and tip of slit is less than 0.5°, then the coma involved can be depressed by adjusting the collimating mirror; the self-rotating of slit less than 10″ can make the slant of spectrum lines less than one pixel; By adjusting the position of CCD, ±10 mm the defocus of collimating mirror is compensated. Then, the observed result is given.