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In this paper, we present energy level, transition configuration, and numerical model of Cr3+/Cr4+:YAG crystal fiber amplifier for the first time, to the best of our knowledge. The rate and power propagation equations of the numerical model are solved and analyzed. The active ion concentration, length of the doped fiber, and pump power are optimized to maximize the bandwidth of the gain spectra. The effect of temperature on the gain spectra is also discussed. It is shown that based on analysis of the absorption spectra and emission spectra, Cr4+:YAG crystal is a three-level system, and the broadband gain of Cr 3+/Cr 4+ :YAG crystal fiber is attributed to the broad emission of Cr4+ ions, especially tetrahedral Cr 4+ in YAG. When excited at 800 nm, optimal fibers have ultrabroad gain spectra in the range of 1.2-1.65 mum, which cover the low-loss windows of the all-wave fiber without absorption peak caused by OH- group.