The invention discloses surface-modified nano-silica and a nano fluid for crude oil recovery. A preparation method of the surface-modified nano-silica includes the steps of allowing nano-silica to have reaction with sulfydryl silane to obtain sulfhydrylated nano-silica; secondly, allowing the sulfhydrylated nano-silica obtained in the first step to have click reaction with monounsaturated fatty acid to obtain the monounsaturated fatty acid surface-modified nano-silica. A preparation method of the nano fluid for crude oil recovery includes: dispersing the surface-modified nano-silica into water to obtain dispersing liquid, and regulating the pH of the dispersing liquid to 8-12 to obtain the nano fluid for crude oil recovery. The monounsaturated fatty acid surface-modified nano-silica particles can disperse into the water without adding a dispersing agent. The nano fluid is simple in preparation process, uniform in dispersing, good in stability, easy to prepare on site, easy to achieve large-scale application, and capable of increasing the recovery ratio of an ultralow-permeability reservoir.