In the present paper, the Mn-doped ZnS quantum dots were synthesized in water, and the MPA was used as the stabilizer. Utilizing the strong quenching effect of Hg2+ to the phosphorescence of the ZnS:Mn quantum dots, the method to detect micro-quantity Hg2+ in water was established by using the quantum dots as the phosphorescence probes. Compared to the fluorescence methods, the phosphorescence has longer lifetime and higher selectivity, and avoids the interference of the fluorescence and the scattering light. The result showed that under the optimum conditions, the relationship between the ΔP/P and Hg2+ concentration was linearity which was ruled by the Stern-Volmer equation while the Hg2+ concentration was between 1.0×10-5 and 1.0×10-3 mol·L-1, and the linear correlation coefficient was 0.998 8. The recovery of the method was between 85.2% and 106.1%, the RSD was 3.46%, and the detection limit was 9.7×10-6 mol·L-1.The mechanism of quenching of phosphorescence was discussed. The interferences of some metal ions could be effectively eliminated by adding appropriate masking agents in the solution.