It is theoretically demonstated that a design of flat-field grazing-incidence spectrograph for soft X-ray is practicable through the numeral calculations of the light path function of the flat-field grating-incidence concave grating. The flexibility of use of the flat-field grating with variable spacing grooves are improved. Furthermore, under the condition of keeping the flat-field focusing, the light flux of the spectrograph can be increased about one order of magnitude by cancelling the entrance slit of the spectrograph ．