Conventional water drive methods have poor effect on ultra-low permeability reservoirs. Imbibition is an important mechanism to improve the recovery of ultra-low permeability reservoir. Nanofluid can improve oil–water properties and enter micro-nano pores, which is one of the ideal reagents to enhance the effect of imbibition oil recovery in ultra-low permeability reservoirs. Through contact Angle test and imbibition experiment, this paper compares the effectiveness of nanofluid to enhance oil recovery, studies the influence of various factors on oil recovery, and uses nuclear magnetic resonance technology to explore the difference of enhanced imbibition pore production of cores with different wettability. The results show that nanofluid can enhance the core’s wettability of the water phase, and 0.1% nanofluid can improve the wettability by 48.3%. Nanofluid can improve the imbibition recovery. Compared with formation water, 0.1% nanofluid increase by 5.5% and 3.6% compared with surfactant with the same concentration. With the increase of concentration, the effect of nanofluid enhanced imbibition oil recovery becomes better and more obvious for water wet reservoirs. For oil wet core and water wet core, there are obvious differences in pore production law. The oil wet core mainly produces crude oil in pores of 0.5 < T2 ≤ 36 ms, and the water wet core mainly produces crude oil in pores of T2 ≥ 13 ms. This paper has reference significance for improving imbibition oil production and further improving oil recovery of ultra-low permeability reservoir.