A two-phase thermal method for the preparation of cadmium sulfide quantum dots where a cadmium source, a sulfur source and a capping agent are heated in water and water-insoluble organic solvents forming a two-phase system. By means of varying reaction time, cadmium sulfide quantum dots of different sizes can be prepared. Quantum dots already obtained can be used as crystal seeds in the reaction of newly added reaction precursor to give larger sized quantum dots with a size distribution similar to that of the initial crystal seeds so as to realize the control of quantum dot size. The quantum dots obtained in the present invention have relatively narrow size distribution, and emit royal purple or blue light under ultraviolet lamp, with a photoluminescence quantum efficiency of from 3 to 60%.