A small sized flat field grating spectrograph was developed by using a varied spacing concave grating with a nominal groove spacing of 1/1200mm and a radius of curvature of 5649 mm. The incidence angle and the object distance of the spectrograph are 87.45° and 155 mm respectively. The spectrograph was used for test in "XINGGUANG Ⅱ" laser system, and the experimental results showed that the spectral resolution of this spectrograph was better than 0.015 nm for time integrating detection with film.