Abstract: In order to improve the mechanical properties of K9 optical glass, which used in some special fields such as heavy pressure and drastic temperature changing, etc, and ensure the optical properties can meet the requirements of precision optical instruments, the chemical strengthening technology for K9 optical glasses was studied. Based on the theory of crack propagation during the fracture process of brittle materials, the calculation model of stress intensity factor for chemical strengthened K9 optical glasses was formulated. The relations between glass surface stress, surface micro-crack depth and toughness were discussed, and several problems in the chemical strengthening process needed to attention were pointed out. By experimental research, the effects of strengthening temperature and time on the strength, surface stress and stress depth of K9 optical glasses were analyzed. The chemical strengthening process for K9 optical glass was optimized, the strengthening temperature is 400 ℃ and the strengthening time is 40 h. The measured results of mechanics and optical properties for K9 optical glasses with size of 220 mm×110 mm×22 mm are obtained. The measured results show that the surface stress is 500 MPa, the stress depth is about 50 μm, the bending strength is increasing more than 3.5 times, and the optical discrimination and transmittance show no obvious changing.