We report a universal approach based on the surface plasmon resonances (SPRs) attained in filamentation in water doped with gold nanoparticles for enhancing the nonlinear refractive index. The filament-induced supercontinuum spectrum in water overlaps with SPRs of gold nanospheres, which further leads to a modification on the Kerr nonlinear refractive index. In our experiment, the measured nonlinear refractive index (n₂) in water doped with gold nanoparticles increases by six times, as compared with that in pure water. Such enhancement may be useful for filament-induced nonlinear applications with modest incident intensity.