Flat-field concave diffraction grating is the key component of a portable grating spectrometer, which integrates three optical properties: dispersion, imaging, and flat spectral image in a single device. In recent years, fabricating concave grating has attracted much attention. However, the distance between two exposure light sources in the fabrication light system are often short, which makes it difficult to build the fabrication structure, sometimes even impossible. In order to solve this problem, two methods have been adopted. One is using special microscope objectives, but it reduces the system’s ability of eliminating of aberrations. The other way is building spatial filters respectively, but this adds the difficulty of the system installing and adjustment, furthermore, it makes it impossible to fabricate wide spectrum FCDG. In this paper, a method to increase the distance between the two exposure light sources is proposed by using a compensation mirror. The use of compensation mirror can not only reduce the difficulty of fabricating grating, but also achieve results almost as better as that of the two original exposure points. We calculated the two new exposure points position with Matlab, and then performed system modeling, simulation and optimization in the Zemax software. A theoretical analysis is given to show that the proposed method can ensure the spectral image quality and greatly reduce the fabrication difficulty.