Nitrite wildly exists in natural environment, with excess nitrite that may cause methemoglobin or Blue baby syndrome. This study achieves a one-pot hydrothermal synthesis of polyethyleneimine-capped copper nanoclusters which based on CuSO₄ and PEI for detection of nitrite. Base on their formation of ground-state complex and fluorescence quenching. The limit of detection was 2 μM in the linear range of 0~1000 μM, which can detect the trace of nitrite.