•We synthesized water-soluble striking fluorescent Cu nanoclusters (Cu NCs) by a simple one-pot chemical reduction method.•This proposed synthetic strategy does not need without any auxiliary stabilizer, organic agent or strong reducing agents.•The prepared Cu NCs show great stability and biocompatible which is essential for construction assays for biological systems.•The nitrite ions fluorescent sensor was successfully constructed based on the prepared Cu NCs. The instant and on-site detection of trace aqueous nitrite ions still remains a challenge for environmental monitoring and protection. In this work, we synthesized water-soluble striking fluorescent Cu nanoclusters (Cu NCs) by a green, simple, one-pot and stabilizer-free chemical reduction method. The fluorescence (FL) of the obtained Cu NCs can be selectively and efficiently quenched by the nitrite ions through the oxidation of Cu (0) atoms to Cu (I) in the core of Cu NCs. Under optimal conditions, two excellent linear relationships between the FL quenching degree of the Cu NCs and the concentrations of nitrite ions in the range of 0.0125–125μM and 125–5000μM with a low detection limit of 3.6nM were obtained. More importantly, the portable paper-based sensor has been explored for visual detection of trace nitrite ions in aqueous solution by the appearance of different degrees of blue FL under a UV lamp. The very simple and effective strategy reported here should facilitate the development of portable and reliable FL chemosensors for nitrite ions in pollution control.