Characteristic gamma rays of 100 keV to about 6 MeV from different elements stimulated by neutrons have been applied to diagnosis biological imaging. In this paper, BGO detectors are used for the spectrum simulation. Signals from a single crystal and after correction are both obtained, and the energy spectrum summing adjacent signals seems possible to achieve excellent energy resolution for such high-energy photons. Some ideal suppositions are introduced and some other information, such as spatial resolution and difficult electronics, should be considered after this correction.