Cd-Zn quantum dots (QDs) modified with thiourea was firstly prepared asthe cores, then the cores were coated with polyacrylic acid with PVP K-30 as the stabilizer in water-soluble reaction system. The water-soluble fluorescence QDs were the same size and stable. The water-soluble QDs were characterized by fluorescence emission spectra, infrared spectra (IR), and transmission electron microscopy（TEM）. The effects of the nuclear QDs with different concentration under polyacrylic acid solution on the optical properties of QDs were also studied. The results showed that the nanoparticles of QDs modified with polyacrylic acid have more uniform particle distribution, and the main peak was blueshifted from 548 to 448 nm. CO and C—O stretching vibration absorption peaks were 2 092.8 and 1 384.3 cm-1 in infrared spectra, amide bond of CO stretching vibration absorption peaks were 1 644.5 cm-1. The best concentration of nuclear QDs under polyacrylic acid solution was 2.67 mg·mL-1. QDs prepared by this simple method have good stability and strong fluorescence intensity. The approach introduced in this paper will lay a solid foundation for biological markers and applications of QDs in the future.