Abstract: This paper aims to the problems such as the unclear mechanisms behind fracturing fluid imbibition (retention) and its impact on oil recovery and reservoir damage in Fuxing area, as well as the undefined limits for imbibition time and undetermined direction for optimization of the efficient fracturing fluid systems. Through various imbibition experiments, comparative analyses of pore size, permeability, and NMR T₂ spectra before and after imbibition, and molecular dynamics simulations of interactions between fracturing fluids and shale oil reservoirs, the paper systematically investigates the mechanism of fracturing fluid imbibition in shale oil reservoirs. The results indicate that the porosity, development degree of micropores, total clay mineral content, and mixed-layer illite-smectite (I/S) content can significantly affect the effectiveness of imbibition oil recovery; higher porosity, higher development degree of micropores, and higher contents of clay minerals, quartz, and I/S are more favorable for imbibition oil recovery. Overall, imbibition can enhance pore expansion, permeability, and oil recovery in the Fuxing shale oil reservoir, and these performances can be improved by 70 % or more when using effective fracturing fluids. Fracturing fluids containing nonionic functional groups such as oxyethylene have a significant effect on the overall pore size distribution of the reservoir and can microscopically disperse shale oil molecules and strip them from the organic pore walls. Reservoir damage is minimized by ensuring a imbibition time of 239 h, and the use of fracturing fluids containing nonionic functional groups like oxyethylene can further reduces reservoir damage and enhance imbibition oil recovery.