Water-soluble CdZnTe quantum dots (QDs) were firstly synthesized by using 3-mercaptopropionic acid (MPA) as modifier in aqueous phase. Then, CdZnTe quantum dots were capped with polyethyleneimine (PEI) with electrostatic interaction and condensation reaction between carboxyl group on the surface of QDs and amino group of PEI, and several metal ions were tested to check if they affect the fluorescence properties of CdZnTe QDs in the paper. A novel method to detect trace Pb2+ has been developed based on the selective fluorescence quenching Pb2+ to PEI-CdZnTe QDs. Fluorescence quenching effect of PEI-CdZnTe QDs had been studied by increasing the Pb2+ concentration. The study results show that fluorescence quenching process of Pb2+ can be described well by Stern-Volmer fluorescence quenching equation. There is a good linear relationship between the fluorescence intensity F0/F and the concentration of Pb2+ when the concentration of Pb2+ is in the ranges of 0.05~3.0 μg·mL-1. The linear correlation coefficient and the detection limits are 0.998 8 and 0.01 μg·mL-1, respectively. The proposed fluorescence method of detection is simple, rapid and sensitive, which has been successfully applied to the detection of Pb2+ in tap water.