Due to long-term freeze-thaw cycling in high-altitude cold regions, rock strength deteriorates, thereby compromising the long-term stability of engineering structures. Granite along the Sichuan-Xizang Railway was selected as the research object, and triaxial unloading tests were conducted on granite specimens subjected to different numbers of freeze-thaw cycles. Based on discrete element numerical simulation, a frost heave numerical model that accounts for the swelling effect of water particles was established. By systematically analyzing the evolution of stress-strain curves, micro-crack propagation, force chain evolution, displacement cloud distribution, and energy transformation characteristics during the triaxial unloading process of freeze-thawed rock specimens, the damage evolution law was revealed. The results show as follows. Freeze-thaw action causes a synchronous decline in the macroscopic mechanical parameters of the rock specimens. After 60 cycles, the measured cohesion, peak stress, and elastic modulus decrease by 56.15%, 59.5%, and 60.2%, respectively. Freeze-thaw damage originates from the initiation and propagation of micro-cracks. The failure mode gradually evolves from shear-dominated to tensile-shear composite, with particle displacement intensifying from the surface inward, ultimately leading to macroscopic spalling. The micro-crack evolution in loaded specimens exhibits a three-stage characteristic of “slow-steep-gentle”. As the number of freeze-thaw cycles increases to 20, 40, and 60, the initiation stress of tensile cracks decreases by 49.9%, 97%, and 97.9%, respectively, while that of shear cracks decreases by 48.8%, 96.6%, and 97.3%, respectively. The initiation of new cracks is suppressed, and energy becomes more concentrated on the connection and propagation of existing cracks. The proportion of elastic strain energy decreases from the initial 92.8% to 76.47% after 60 freeze-thaw cycles, indicating that freeze-thaw action significantly weakens the energy storage capacity of the rock, thereby revealing the essential cause of its strength loss from the perspective of energy mechanism.