Abstract: This paper aims to clarify the strength degradation and damage mechanism of sandstones with different grain sizes under freeze-thaw cycles. It uses both coarse-and fine-grained sandstones to conduct uniaxial compressive strength, nuclear magnetic resonance, and electron microscopy scanning tests under 0-25 freeze-thaw cycles. The conclusions include the following: The degradation process of uniaxial compressive strength of two types of sandstones caused by freeze-thaw cycles follows a two-stage pattern. The first stage is 0-15 freeze-thaw cycles, and the second stage is 15-25 freeze-thaw cycles. The strength degradation rate in the first stage is faster than that in the second stage, and the degradation rate of coarse-grained sandstones is lower than that of fine-grained sandstones. Corresponding to the deterioration process of uniaxial compressive strength, the porosity of both types of sandstones increases in two stages. The porosity growth rate in the first stage is faster than that in the second stage, and the increase rate of coarse-grained sandstone is lower than that of fine-grained sandstone. Based on the analysis of pore evolution and pore structure, there are two types of freeze-thaw damage mechanisms in sandstones with different grain sizes: (1)Frost heave force leads to seepage of unfrozen water in connected pores. Due to the larger pore size and stronger permeability of coarse-grained sandstones, the pore development rate of coarse-grained sandstones is lower than that of fine-grained sandstones. (2)The development trend of micropores controls the freeze-thaw damage of the two sandstones. Due to the decrease in the number of newly generated micropores and the slowing down of the frost heave rate of micropores, the pore development rate in the first stage is faster than that in the second stage. The research results provide a reference for the study on damage of rocks with different grain sizes after freeze-thaw cycles.