Carbon dots (CDs) attract great interests from scientists for their low toxicity and biocompatibility properties and their important applications in the fields of photocatalysts, batteries, bio-images and supercapacitors. Most strategies of making CDs contain several steps, which can be a time-consuming and costly procedure. In this study, nitrogen-doped CDs have been prepared by a one-step hydrothermal strategy with sodium citrate and ethylenediamine as precursors. It is found that fluorescence intensity of CDs is enhanced with the increased content of doped nitrogen, which implies that nitrogen-doped element plays an important role to improve the fluorescence intensity of CDs. Most importantly, those CDs yielded high selectivity of Fe3+, with good linearity, precision and accuracy. Hence, the as-prepared nitrogen-doped CDs could be used as probes for quantitative analysis of Fe3+ in environmental applications.