Bi-doped CsI crystals exhibited near-infrared ultra-broadband photoluminescence around 1216 nm and 1560 nm, depending on the bismuth doping levels, which were ascribed to Bi+ and Bi2 + centers, respectively. The crystal chemistry of the Bi3+ to Bi+ reduction and Bi2 + dimer formation in CsI lattice were investigated. Thermal treatments including annealing and quenching were carried out to study the thermal behaviors of the two emission bands. The evolution of absorption and emission spectra of Bi:CsI crystals indicating the Bi-aggregation and valence conversions under thermal activation. The process of Bi aggregation was observed to be a second-order reaction with activation energy of 0.33 eV. Bi2 + was identified as the origin of the 1560 nm emission band with ESR spectra. A simple lattice structure diagram was developed to illustrate the physical processes in Bi:CsI crystals induced by thermal activation.