In order to explore spontaneous imbibition mechanism of different pore structures and its influence on the imbibition process in the low-permeability tight reservoir，the pore structure of the low-permeability tight sandstone reservoirs in Chang6 member in X study area of eastern Ordos Basin was classified. On this basis，the influence of various pore structures on the imbibition characteristics and efficiency of oil displacement was researched through mercury intrusion，SEM and NMR and other analysis methods taking Bond index and imbibition efficiency in dimensionless time as the evaluation index. The experiment results show that there are two types of pore combination：mid-large pore and micro-pore and fracture. The proportion of the mid-large pore throat decides oil displacement effect of the imbibition in cores having different pore structure combinations，and the residual oil mainly remains in the micro-pore throat. The core having mid-large pore is characterized by high proportion of large pore throat，higher oil displacement efficiency of the imbibition in formation water and surfactant，weak effect from capillary pressure during the imbibition process and an evident change in oil displacement efficiency and the way of imbibition by surfactant. The core with micro-pore and fracture is characterized by low proportion of large pore throat，strong effect from capillary pressure and relatively poor imbibition efficiency in formation water and surfactant. Though the surfactant can significantly change the way of imbibition and improve oil displacement efficiency，a part of the residual oil still remains in the micro-pore throat.