Yttrium aluminum garnet (YAG) crystal is an important laser host 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 characteristics and the evolution mechanism of defect structure of 111, 110 and 100 crystal orientations from 300 to 2200 K are 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-dependent 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 is verified by nanoindentation experiments, which also indicates that the 110 crystal orientation has the highest tensile strength. The simulation results further 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. At 2200 K, YAG exhibits continuous plastic deformation after reaching a yield stress of 6 GPa. With increasing temperature, the elastic modulus decreases, and the mechanical anisotropy weakens or disappears. Based on the stress-strain curves, the empirical formulas describing the variation of fracture strain, ultimate stress, and fracture energy of YAG with temperature are provided. The defect evolution characteristics of YAG vary with temperatures. Under tensile deformation, YAG crystals form defects such as micropores and microcracks. At elevated temperatures, lattice slip becomes more pronounced, leading to the formation of dislocations and stacking faults. Near the melting point, plastic deformation is likely to produce extensive disordered glassy structure. This study provides an atomic-scale theoretical basis for optimizing the YAG crystal growth process. It has guiding significance for large-size, high-quality YAG crystal optical elements.