TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The cadmium precursor solution is prepared by dissolving CdO or cadmium salt (as cadmium precursor) in mixture of long chain fatty acid and liquid paraffin at 150 degrees C, with the mole ratio of cadmium precursor to long chain fatty acid at 1:1 to 1:5, and the concentration of the cadmium precursor at 50-400 mmole/L. The sulfur precursor solution is prepared by dissolving sulfur powder in liquid paraffin at 120 degrees C, with the concentration of the sulfur precursor at 5-40 mmole/L. The CdS quantum dot solution is prepared by reacting cadmium precursor solution and sulfur precursor solution at 2:1 mole ratio at 180-240 degrees C for 1-30 minutes, then cooling to room temperature. The ZnS shell precursor solution is prepared by dissolving zinc salt and (TMS)2S in mixture of phosphine and liquid paraffin, with the volume ratio of phosphines to liquid paraffin at 1:3, the concentration of zinc salt at 25-200 mmole/L, the concentration of (TMS)2S at 25-200 mmole/L, then reacting ultrasonically. A CdS and CdS/ZnS core-shell quantum dot is obtained by adding ZnS shell precursor solution into CdS quantum dot solution dropwise at 100-160 degrees C, reducing temperature to 60-100 degrees C, preserving heat for 30-90 minutes, then cooling to room temperature. The purifying step of the obtained quantum dot solution comprises precipitating the quantum dot solution with a precipitant (including methanol, alcohol, propanol or acetone), centrifuging, removing supernatant liquid, dissolving the flocculent precipitate in solvent (including n-hexane, cyclohexane, chloroform, tetrahydrofuran or toluene), and centrifuging again to obtain uniformly dispersed quantum dot. Preferred Components: The cadmium salt is cadmium stearate, cadmium acetate or cadmium oxalate. The long chain fatty acid is oleic acid, stearic acid, palmic acid or olive oil. The zinc salt is zinc stearate, zinc acetate or zinc oxalate. The phosphine is trioctylphosphine, tributylphosphine, triphenylphosphine or myristylphosphoric acid. Preferred Parameter: The CdS quantum dot has a size distribution of less than 10%, a fluorescence emission peak of 360-460 nm, a peak width at half-height of the fluorescence emission peak of less than 30 nm, and a fluorescence quantum yield of 20-40%. The CdS and CdS/ZnS core-shell quantum dot has a size distribution of less than 10%, a fluorescence emission peak of 370-485 nm, a peak width at half-height of the fluorescence emission peak of less than 30 nm, and a fluorescence quantum yield of 30-60%.