Abstract: To study the evolution rule of pore structure in backfill material under freeze-thaw cycles, the nuclear magnetic resonance (NMR) technique was used to scan the meso-structure damage after 0, 50, 100 and 200 freeze-thaw cycles to obtain the relaxation time T₂ spectrum of NMR. The laws of pore evolution and the mechanism of meso structure damage were analyzed. The results show that the variation of T₂ distribution curves reflects the evolution and expansion characteristics of the pores in the samples under freeze-thaw cycles; with the increase of freeze-thaw cycles, the T₂ spectral area which characterizes the porosity of the materials increases rapidly at the initial stage and decreases at the later stage, and the proportion of small pores increases with the number of cycles. With the increase of cycles, the ratio of macro-pores increases with the increase of the number of times. Through the discussion of the evolution mechanism of pore structure, it is pointed out that the joint failure due to the combined action of osmotic force and frost heave force in freeze-thaw cycles is the fundamental reason for the formation of the connecting fracture.