Biocompatible amphiphilic block copolymer dextran-b-poly(L-lactide) was synthesized by the end-to-end coupling of amino-terminated dextran and acryloyl-terminated poly(L-lactide). The dextran block copolymer was used to rapidly prepare nanoparticles encapsulating beta-carotene through flash nano-precipitation. The nanoparticle size can be tuned by different solvent ratios and stream velocities. The nanoparticles could be stably preserved for more than half a month, and an "anti-Ostwald" phenomenon was observed in the first week. The results indicate that flash nanoprecipitation is a promising method to prepare polysaccharide nanoparticles as drug carriers.