In this paper, the pore structure characteristics of CPB specimens were characterized using nuclear magnetic resonance (NMR) and scanning electronic microscopy (SEM) techniques, and the relationship between the unconfined compressive strength (UCS) and the pore characteristics of cemented paste backfill (CPB) was investigated. The results showed that the pores in CPB specimens were mainly small, “non-damaging” ones, which accounted for about 70 vol% of all pores in CPB specimens. Although the “damaging” and “multi-damaging” pores only accounted for 3.93 vol% and 6.99 vol% of all pores respectively, they had significant impacts on the UCS of CPB specimens. With the increase of curing time, the porosity of CPB specimens decreased gradually (i.e. the average porosities of CPB specimens at the curing time of 3 d, 14 d and 28 d were 11.07 vol%, 8.58 vol% and 7.09 vol%, respectively). Besides, the porosity of CPB specimens also decreased with the increase of solid concentration of CPB slurry. On the basis of fitting analysis, it could be observed that there was a negatively exponential relationship between the UCS and the porosity of CPB specimens and a linearly inverse relationship between the UCS and the fractal dimension. Our study demonstrates that it is feasible to study the relationship between mechanical properties and pore characteristics of CPB specimens by using the NMR technique.