The imbibition oil recovery uses the capillary pressure of low-permeability reservoirs to enhance the oil-water displacement between matrices and fractures，thereby effectively improving the recovery factor. At present，there is a lack of systematic studies on the basic laws of imbibition oil recovery，and its main controlling factors are not clear. A high-temperature and high-pressure imbibition instrument was adopted，with the imbibition recovery factor as an indicator，to systematically evaluate the influence of seven parameters of three types，including reservoir characteristics，fluid properties，and boundary conditions，on the spontaneous imbibition of cores. The weight of each influencing factor was analyzed by grey relational analysis. The results show that the imbibition recovery of low-permeability reservoirs decreases with the increase in core permeability and length. Lower oil-water interfacial tension and stronger hydrophilicity of rock surfaces lead to a higher imbibition recovery. A higher temperature results in lower viscosity of crude oil and stronger imbibition effect，with the imbibition recovery on the increase. The imbibition recovery of cores open at both ends is higher than that of cores open on the side. More fractures bring about a higher imbibition recovery. The weight of each influencing factor in descending order is rock wettability，oil-water interfacial tension，number of fractures，reservoir temperature，end-face opening position，core permeability，and core length. Among them，oil-water interfacial tension，rock wettability，and number of fractures are the main controlling factors of imbibition oil recovery as their correlations with imbibition recovery are all above 0.95.