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Gaomiaozi (GMZ) bentonite is a potential buffer/backfill material for a deep geological disposal of high-level radioactive waste. It has a wide pore size distribution (PSD) with sizes ranging from several nanometers to more than one hundred microns. Thus, properly characterizing the pore structures of GMZ bentonite is a challenging issue. In this study, pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), and scanning electron microscopy (SEM) were used to investigate the PSD of GMZ bentonite. The results indicate that each method has its limitation, and a combined use of PCP and RCP is suitable to obtain the full-scale PSD of GMZ bentonite. Moreover, we also compared the full-scale PSD with nuclear magnetic resonance (NMR) result. It is found that there is no significant difference in the range of PSD characterization between NMR and mercury intrusion method (PCP and RCP). However, in a certain range, the detection accuracy of NMR is higher than that of mercury injection method. Finally, permeability prediction based on PCP and SEM data was conducted, and both of the two methods were found to be able to predict the permeability. The combined method is effective to obtain the full-scale PSD of GMZ bentonite, which is the key to estimation of the sealing ability of bentonite buffer.