Monodisperse water-soluble LaF3: Yb3+/Er3+ nanocrystals (NCs) have been successfully fabricated via a fast, facile, and environmentally-friendly polyol process with polyvinylpyrrolidone (PVP) as an amphiphilic surfactant. The obtained NCs with a small size of 18 nm can be well dispersed in hydrophilic solutions. Meanwhile, their upconversion (UC) luminescence intensity was increased through Ca2+ doping due to the decrease of symmetry around rare earth ions.