A coaxial capillary array microreactor was designed and constructed to prepare fluorescent polymer dots (Pdots) by a nanoprecipitation method. Poly(9,9-dioctylfluorene-co-benzothiadiazole) (PFBT) was used as polymer, tetrahydrofuran (THF) as solvent, deionized water as anti-solvent and polystyrene-maleic anhydride copolymer (PSMA) as stabilizer in the experiments. The prepared Pdots were characterized by dynamic light scattering (DLS). The results showed that the accumulation and blockage of products were avoided at the tip of the injection nozzle in the microreactor due to the middle solvent stream which changed the mixing process between polymer solution and antisolvent. The microreactor kept on running well for a long time even at the high concentration of polymer solution of 500 mg/L. The size of the nanoparticles can be adjusted by varying the concentration of the polymer solution and the flow rates of the solution, solvent, and antisolvent. The diameters of the prepared nanoparticles were decreased down to 13 nm at the concentration of the PFBT solution of 50 mg/L and the flow ratio of deionized water to polymer solution of 750:1. The microreactor can be used to prepare continually polymer nanoparticles with controllable size.