The cysteamine (CA) modified CdTe (CA-CdTe) quantum dots (QDs) with different particle sizes were prepared by hydrothermal method. And the fluorescence response of CA-CdTe QDs to Co2+ was measured and analysed. The results indicate that Co2+ can quench the fluorescence of CA-CdTe QDs. With the increase of Co2+ concentration, the fluorescence intensity of CA-CdTe QDs was gradually decreased. When Co2+ concentration is in the range of 0.3×10-5～33×10-5 mol·L-1, the Stern-Volmer quenching equation of the interaction between Co2+ and CA-CdTe QDs with particle size 3.60 nm is obtained, and the detection limit is 6.547×10-7 mol·L-1. The method of quantitative detection of Co2+ based on the CA-CdTe QDs fluorescence analysis method, is simple, rapid, and with low detection limits, which provides a theoretical basis for detecting Co2+ in real water samples.