The minitype flat-field holographic concave grating spectrograph equipped with CCD detectors are widely used for spectral analysis. They are accepted for some remarkable advantages, such as compact structure and rapid and efficient testing process. However, restricted by imaging distance of the spectrometer, it is difficult to improve the spectral resolution greatly just by optimization of the holographic concave grating. A design method of double-grating minitype flat-field holographic concave grating spectrograph is proposed. The single grating in conventional spectrograph is replaced by two gratings which are equipped with the same geometry. A double grating flat-field spectrograph with a wavelength range from 400 nm to 1000 nm is designed. The calculation results show that the resolution of the newly designed spectrograph can be almost two and a half times as great as the conventional spectrograph. The light throughput efficiency can also be greatly improved, which is demonstrated by analyzing diffraction efficiency of the grating. The double-grating minitype flat-field holographic concave grating spectrograph is developed and adjusted. The experimental results agree with the theoretical calculations very well.