In order to study the mechanism of oil displacement by imbibition and the influencing factors of imbibition process in low permeability tight sandstone reservoir, the compact sandstone reservoir of Chang 8 reservoir section in H block of Ordos Basin was taken as the research object. The pore type, pore radius, throat radius and pore throat ratio were analyzed by means of casting slice, scanning electron microscopy and constant velocity mercury injection. The characteristics and influencing factors of spontaneous imbibition flooding were studied by imbibition experiment and nuclear magnetic resonance experiment. The results show that the pore types of tight sandstone reservoirs in the study area are mainly intergranular pore, intragranular dissolved pore and intercrystalline pore, which are mainly divided into medium-large pore and micro-pore. The pore radius of these two types of pore is not very different, but the number of pore, throat radius and pore-throat ratio are quite different. In the initial stage of spontaneous imbibition of tight sandstone cores, the imbibition rate is faster, and then gradually decreases. The effect of spontaneous imbibition and displacement of medium and large porous cores is better than that of micro-porous cores. With the increase of water wettability on the surface of tight sandstone cores, the spontaneous imbibition recovery rate increases gradually; and with the decrease of interfacial tension of imbibition, the spontaneous imbibition recovery rate increases first and then decrease. Therefore, in practical application, the appropriate interfacial tension should be applied to make the spontaneous imbibition recovery rate maximize.