Interests in using nanofluids for enhanced oil recovery (EOR) applications has been increasing. Herein, a novel surfactant-free water-based nanofluid for EOR was constructed using active silica nanoparticles. Active silica nanoparticles were synthesized via condensation of hexanedioic acid with the OH group of silica. Water-based nanofluid was obtained by transforming carboxyl into carboxylate on the surface of active silica nanoparticles in water. The particle size of the active silica nanoparticles in water ranged from 10 to 20 nm. The interfacial activity of the nanoparticles was endowed through the shape change of the active silica nanoparticle surface groups to minimize their interface energy. The morphology and surface components of the active silica nanoparticles were characterized by transmission electron microscopy and Fourier transform infrared spectroscopy. The interfacial activity of the active silica nanoparticles was proved through interfacial tension and interfacial dilational modulus measurements. Active silica nanoparticles exhibited a stronger ability to reduce interfacial tension and enhance the interfacial film strength than silica nanoparticles. Contact angle measurements showed that this nanofluid exhibited excellent capabilities of oil displacement from a solid surface and wettability alteration. Spontaneous imbibition tests of ultralow permeability cores using different liquid phases (active silica nanofluid, silica nanofluid and brine) were conducted. Oil recovery using active silica nanofluid was higher than that of using silica nanofluid or brine. Active silica nanofluid at a low concentration could display an equal EOR efficiency with highly concentrated silica nanofluid. These results indicated the possible application of the proposed active silica water-based nanofluid in EOR. This preparation method could be used to prepare surfactant-free active nanofluids, and the surfactant-free active nanofluid shows great potential for EOR applications.