A simple and low-cost method is reported here for synthesizing polymer dots (PDs) using branched polyethyleneimine and peryleneimide derivatives as precursors heated in dimethylacetamide. The as-prepared PDs can be well-dispersed in water and show excitation-dependent fluorescence, stable fluorescence over a wide range (pH = 5.0 - 9.0), and high photostability. It is demonstrated that the prepared PDs can be used as a novel fluorescent sensing platform for sensitive and selective detection of Fe (III) ions. The fluorescent PDs may be applied to promising applications in chemical sensors for metal ions, as well as biological imaging or biological labeling for their excellent fluorescence properties.