There are abundant defect structures in lithium niobate crystals, mainly including VLi, NbLi4+, VNb and VO point defects. The existence of defects greatly affects the properties of lithium niobate crystal(i.e., piezoelectric, electro-optic, ferroelectric, photorefractive, nonlinear optical properties and laser damage threshold) as well as the performance of SAW, electro-optic modulator, acousto-optic modulator, temperature/pressure/acceleration sensor and other devices. The main process of defect formation in lithium niobate crystal can be attributed to the evolution of mesoscale clusters centered on O2?傆b ions. It is thusimportant to understand the evolution mechanism of mesoscale clusters in the defect formation process of lithium niobate crystal for defect control. This paper was to investigate the dynamic evolution process and formation mechanism of defects in lithium niobate crystal from the aspects of defect types, formation energy and mesoscale cluster model. The hybridization and rehybridization process caused by defects in lithium niobate crystal structure was analyzed, and the coupling of multiple degrees of freedom was considered, which contributes to the defect control, rapid growth and performance control of lithium niobate crystal. In addition, the correlation between the defects of lithium niobate crystal and its properties as well asfunctions was also analyzed