To address the problem of bulk damage of K9 optical components induced by laser irradiation, the on-line imaging technology is used to obtain front- and side-view images of laser-induced bulk damage. The characteristics of filament damage in bulk damage, influence of nucleation damage in filament damage, and effect of crack on damage growth are investigated. Results indicate that when the irradiation energy is larger than the bulk damage threshold, the bulk damage appear in the forms of filament damage, nucleation damage, and damage growth. However, only the nucleation damage site closest to the direction of the incident laser beam continues to grow under subsequent laser irradiation. Moreover, in the damage-growth process, the cracks of nucleation damage propagate along the longitudinal and reverse directions of propagation of the laser beam. Further, under the same laser irradiation, the growth of damage area strongly depends on the length of the filament damage site. Moreover, the damage-growth coefficient of the damage site observed in the side direction is larger than that observed in the front direction. These results can provide a reference for investigating bulk damage and damage growth mechanisms under subsequent laser irradiation of K9 optical components.