Abstract: Active faults are widely distributed on the Qinghai-Tibet Plateau, and there are a large number of fissures in rock masses. Under freeze-thaw action, fissured rock masses are prone to further damage and deterioration, leading to the instability of rock mass structures. To explore the mechanical properties, failure modes, and microscopic damage of intact and fissured rock masses under freeze-thaw action, taking the conglomerate from Lagangcun landslide as the research object, rock samples with prefabricated fissures of 2 cm and 4 cm were prepared. Triaxial unloading tests were carried out on intact and fissured rock samples after 0, 20, 40, and 60 freeze-thaw cycles. The microscopic pore structure, porosity, permeability, and grayscale value of freeze-thawed rock samples were obtained by CT scanning, and the evolutionary characteristics of microscopic freeze-thaw damage and mechanical properties were analyzed. Based on the grayscale value of CT images, the freeze-thaw damage variable was defined. By introducing damage mechanics theory and Weibull distribution function, a constitutive model considering freeze-thaw damage was established. The following results were obtained: Freeze-thaw action causes a decrease in the peak stress of conglomerate, while the presence of a fissure weakens the strength of conglomerate. The deeper the prefabricated fissure, the more significant the weakening effect. Under freeze-thaw action, the failure mode of fissured conglomerate transforms from a single tensile failure to a complex tensile-shear mixed failure. Furthermore, the number of shear and tensile failure surfaces grows as the freeze-thaw cycle increases. Freeze-thaw action leads to a reduction in the number of pores, an increase in the number of throats, and an enhancement in pore connectivity, while the average grayscale value decreases with the increase in freeze-thaw cycle. After 60 cycles, the permeability of intact samples and those with 2 cm and 4 cm prefabricated fissures rises by 1.87%, 55.56%, and 183.90% respectively, and their grayscale values fall by 43.93, 50.96, and 73.53 respectively. The damage constitutive model considering freeze-thaw damage can accurately characterize the complete evolutionary process of conglomerate from deformation to failure under different freeze-thaw cycles, and can well describe its damage characteristics after freeze-thaw cycling. The research results can provide a reference for exploring the freeze-thaw damage mechanism of fissured rock masses on the Qinghai-Tibet Plateau.