A miniature flat-field spectrophotometer was designed by using the mode of “first absorption then spectroscope”. The multi-channel silicon photodiode array detector and flat-field concave holographic grating were used and the collimating and focusing structure were integrated into one.Based on the selected flat field concave grating， the structure analysis and design of the spectrometer unit was completed.By studying the structure of the miniature flat-field spectrophotometer， a method for the analysis of spectral bandwidth broadening effect and the estimation of wavelength error was proposed.The near-straight spectrum was obtained within the predetermined working band range， and the multi-wavelength synchronous measurement of a single signal beam received could be realized. The working band covered 340 nm~800 nm， and the spectral bandwidth was better than 8 nm.