In recent years, high power solid-state lasers have been developing in the direction of higher average power and larger pulse energy, which puts higher and higher requirements on the size and quality of the laser crystal materials. The preparation of high quality laser crystals with larger size has become one of the important issues to be solved in laser development, so the thermal bonding technology has become an effective technical way to prepare large size laser crystals. Thermal bonding technique, also known as diffusion bonding technique, is a process in which two precise-machined crystal interfaces are pressed together and combined through van der Waals force or hydrogen bond, and then heated to form an integrated uniform crystal by ion thermal diffusion. The preparation of composite crystals by the thermal bonding technology has been widely used in laser field. The composite crystals can not only significantly improve the thermal management of high power lasers and obtain high optical quality lasers, but also is an important way to prepare larger crystals. However, in the process of the bonded crystals preparation, due to the factors such as uneven pressure and uneven distribution of temperature field during heating, the bonding surface will produce bubbles, holes, cracks and other defects, which seriously affect the quality and success rate of the bonded crystals. The Hot Isostatic Pressing (HIP) technique is to place the components to be treated in a closed container, and apply the same pressure to the components and apply high temperature at the same time. Under the action of high temperature and high pressure, the internal defects of the components can be eliminated. By combining HIP technology with the thermal bonding technology and controlling hot isostatic pressing parameters, the internal defects of bonded crystals after sintering can be reduced or eliminated.In this paper, the treatment of large size Yb∶YAG/Yb∶YAG composite laser crystals by hot isostatic pressing technique was studied. The hot isostatic pressing of Yb∶YAG/Yb∶YAG composite laser crystals was carried out under different temperature, time and pressure. The quality of the components after hot isostatic pressing was characterized by bending strength, elastic modulus, Vickers hardness, transmission spectrum and optical uniformity. By means of metallographic microscope, pore elimination and defect closure in bonded crystals were observed, and the mechanism of defect reduction or elimination by HIP was studied by analyzing the change of intergranular stress. The experimental results show that hot isostatic pressing is beneficial to improve the properties of large size and high quality composite crystals at 1 600 ℃ and 200 MPa. After HIP treatment, the bubbles and defects on the bonding surface of the bonded crystal elements are greatly reduced, the bending strength is increased by 185.29%, the Vickers hardness is increased by 16.14%, and the optical transmittance and optical uniformity are significantly improved. After hot isostatic pressing, all the bubbles and defects on the bonding surface of the composite laser crystal are eliminated, and the mechanical and optical properties of the composite laser crystal are obviously improved, and the properties are basically consistent with those of Yb∶YAG single crystal. With the continuous deepening and improvement of the research, it can provide a basis for the performance optimization and wide application of bonded crystals.