Yttrium aluminum garnet (YAG) crystal is an important laser matrix material. Its atomic-scale mechanical characteristics and defect generation mechanism are of great significance for the growth of high-quality large-size crystals. In this study, the tensile mechanical properties, energy property and defect structure evolution mechanism of 111, 110 and 100 crystal orientations at 300~2200 K were revealed by molecular dynamics simulation. It is found that YAG exhibits typical brittle material characteristics at room temperature. The mechanical properties of YAG crystal are anisotropic. The simulation confirms that the elastic modulus of YAG has a significant strain scale effect. When the strain is less than 0.02, the order of elastic modulus is 100 > 110 > 111.When the strain exceeds 0.05, 100 orientation has the largest decrease in elastic modulus, and the order changes to 110 > 111 > 100. This phenomenon was verified by nanoindentation experiments, indicating that the 110 crystal orientation has the highest tensile strength. The simulation results also show that the mechanical properties of YAG are significantly affected by temperature. It shows typical brittle fracture characteristics below 1300 K, and a very narrow plastic deformation zone appears at 1800 K. It shows continuous plastic deformation ability after reaching 6 GPa yield stress at 2200 K. As the temperature increases, the elastic modulus decreases, and the mechanical anisotropy weakens or disappears. According to the strain-stress curve, the empirical formulas of YAG fracture strain, ultimate stress value and fracture energy changing with temperature are given. The defect evolution characteristics of YAG at different temperatures are also different. Under tensile deformation, YAG crystals will form defects such as micropores and microcracks. After increasing the temperature, lattice slip is more likely to occur, forming dislocations and stacking faults. Near the melting point, plastic deformation is likely to occur to produce a large range of disordered glassy structure. This study provides an atomic scale theoretical basis for the optimization of YAG crystal growth process. This has guiding significance for the preparation of large size and high quality YAG crystal optical elements.