Abstract: Experimental studies of spontaneous imbibition mostly focus on wettability and interfacial tension, while the microscopic pore structure as one of the primary factors affecting spontaneous imbibition is usually neglected. Nuclear magnetic resonance (NMR) technology was used to investigate the change of oil and water distribution in tight sandstone during spontaneous imbibition; in addition, constant-speed mercury injection, high pressure mercury injection, nitrogen adsorption and other test methods were adopted to obtain characteristic parameters of microscopic pore such as average pore throat ratio, specific surface area and pore size. On this basis, the effect of microscopic pore structure on spontaneous imbibition was analyzed and studied. Experimental results showed that medium pores achieved the highest recovery rate in the process of spontaneous imbibition; the porosity was correlated less with the imbibition recovery rate while the imbibition recovery rate was proportional to the permeability and reservoir quality; the average pore throat ratio and specific surface area were negatively correlated with the imbibition recovery rate, medium and large throats increased with the specific surface area and the higher pore throat ratio was more adverse to absorption of imbibition fluids and non-wetting phase discharge; the imbibition recovery rate was proportional to the medium pore ratio and the small pore ratio increase was adverse to the improvement of the imbibition recovery rate, because some small pores could not achieve effective imbibition under viscous forces. Therefore, tight sandstones with the highest reservoir permeability and medium pore ratio were more suitable for imbibition oil recovery.