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In this paper, the internal pore structure and mechanical properties of cemented paste backfill (CPB) with different mix proportions are studied by low field nuclear magnetic resonance (NMR) and unconfined compressive strength (UCS) test, and the relationship between UCS, pore structure characteristics and curing time is quantitatively characterized. The following conclusions are obtained: (1) The internal pores of CPB are divided into micropores, secondary pores, and macropores, among which secondary pores account for the largest proportion, accounting for about 70% of the total pores. The proportion of macropores is inversely proportional to the UCS, while the proportion of micropores is proportional to the UCS. (2) The porosity of CPB decreases with the increase of curing time, and UCS increases with the increase of curing time. The curing time has an exponential function relationship with porosity. UCS decreases exponentially with the increase of porosity. (3) A damage model of CPB is established considering the initial pore damage, which is highly consistent with the experimental results. The total damage evolution curve presents an “S'' type distribution. The total damage change rate evolution curve showed a normal distribution.