This paper reports the experiment and theory simulation results of silver substituting cadmium in CdS quantum dots (QDs). The experiment results show that the average particle size of as-prepared samples is about 6.0 nm, and the phase structure is hexagonal wurtzite. Compared with that of pure CdS QDs, the peak of photoluminescence spectrum of CdS QDs the cadmium of which are substituted by silver are red shift from 533 nm to 567 nm, which leads to the luminous colours change from green to yellow. The theoretical simulation results show that the optical property changes of CdS QDs are due to the silver substituting cadmium changing the energy level spacing, the conductivity and the way of electronic transition of CdS crystals.