Nitrogen-doped carbon quantum dots (N-CDs) have become a current focus of chemical research due to their superior photoluminescence (PL) properties and high fluorescence quantum yield (QY). In this paper, we reported a facile, simple and low-cost approach for the synthesis of highly fluorescent and stable N-CDs by hydrothermal treatment of maleic acid and ethylenediamine. The as-prepared N-CDs emitted strong blue fluorescence with an absolute QY of up to 45%, and exhibited pH-independent behavior in a wide pH range from 1 to 13 and good water solubility. In addition, the fluorescence of the N-CDs can be significantly quenched by p-nitrophenol (4-NP) through an electron-transfer-induced dynamic quenching mechanism, leading to 4-NP being sensed optically. The sensor presented a good linearity with 4-NP concentration in the range of 0.10–11 μg mL⁻¹. The detection limit of 22 ng mL⁻¹ 4-NP could be obtained, which is lower than the 60 ng mL⁻¹ limit allowed in drinking water by the U. S. Environmental Protection Agency. The sensor exhibited rapid recognition and distinguished selectivity to 4-NP over other structural analogues of phenols. Moreover, the sensing system was successfully applied for the determination of 4-NP in environmental water samples.