Due to the long-term freeze-thaw cycle of rocks in high-altitude regions, their strength decreases, which affects the long-term stability of the project. Triaxial unloading tests on granite from the Sichuan Tibet Railway after different freeze-thaw cycles are conducted. Based on discrete element numerical simulation, a numerical model of frost heave considering the effect of water particle expansion is 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 samples, the damage evolution laws are revealed. The results indicate that: (1) Freeze-thaw action leads to a synchronous decline in the macroscopic mechanical parameters of the rock samples. After 60 cycles, the measured cohesion, peak stress, and elastic modulus decreased by 62.08%, 59.5%, and 60.2%, respectively. (2) 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. (3) 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. (4) 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, revealing the essential reason for its strength loss from the perspective of energy mechanism.