Amphiphilic polymer nanospheres of poly(3-sulfopropyl methacrylate potassium salt-co-styrene) [P(SPMA-co-St)] were prepared by a simple soap-free emulsion polymerization method and used as efficient water lubrication additives to enhance the antiwear behaviors of the Ti₆Al₄V alloy. The monodisperse and flexible P(SPMA-co-St) bicomponent copolymer nanospheres were synthesized with a controllable manner by adjusting the mass fraction ratio of the monomers, with the hydrophobic polystyrene (PSt) as the hard skeletal carrier component and the hydrophilic PSPMA with a hydration layer structure as the soft lubrication layer in the course of friction. The influences of the monomer concentration, the copolymer nanosphere additive content, the load, and the frequency of the friction conditions on their tribological properties were studied in detail, and a probable antiwear mechanism of the soft/hard-coupled copolymer nanospheres under water lubrication was also proposed. The results show that compared with pure PSt, the P(SPMA-co-St) polymer nanospheres exhibited better antiwear property as an additive for water lubrication, and the friction coefficient and the wear volume first decreased and then increased with the increase of the SPMA content, indicating that the hydrophilic SPMA has a significant effect on lubrication properties owing to its hydration performance. Furthermore, with the increase of polymer nanosphere concentration, the friction coefficient and wear amount also decreased to a stable and low value at a saturation concentration of 1 wt %. The flexible polymer nanospheres with a hydrophilic soft SPMA shell and a rigid PSt core exhibited good friction-reduction and antiwear performance as lubrication additives, indicating their promising and potential applications in water lubrication and biological lubrication.