In order to estimate the potential of surfactant enhancing the imbibition efficiency of shale oil and to investigate its micro-mobilization characteristics in Lucaogou formation of Jimsar Sag，the imbibition efficiency of the anionic surfactant Surf-L in natural shale cores with different wettability and the change of oil saturation in different pores with time was studied by using the low-field nuclear magnetic resonance imaging（LF-NMRI）technique. The results from NMRI T₂ spectrum showed that the 0.1% surfactant Surf-L could effectively improve the core’s imbibition efficiency. For neutral wetting and homogeneity sand core S52， guar gum solution and the Surf-L guar gum solution mainly mobilized the oil in large and middle pores. Among them，the contribution rate of large pores to imbibition efficiency was the highest. The 0.1% Surf-L guar gum solution could effectively mobilize crude oil in middle and small pores，and the final imbibition of 0.1% Surf-L guar gum solution was 6.95% higher than that of guar gum solution. For oil-wetting sandy dolomite core S39，guar gum solution mainly mobilized the crude oil in large pores， while the Surf-L guar gum solution enabled to mobilize the oil in middle and small pores. The imbibition efficiency of Surf-L guar gum solution was 18.89% higher than that of guar gum solution. The imbibition process of Surf-L guar gum solution can be divided into three stages，namely early，middle and late stage. In the early stage of imbibition，the maximum contribution rate supported by middle pores was 61.7% . In the middle and late stage of imbibition，the contribution of large pores gradually increased. The imbibition equilibrium time of guar gum solution was about 51 hours while the imbibition equilibrium time of Surf-L guar gum solution was about 100 hours. Regardless of neutral wetting or oil-wetting cores，surfactant can enhance imbibition oil recovery of shale reservoir by mobilizing the oil in the middle and small pores effectively compared with the guar gum solution.