By using CdSe/ZnS quantum dots (QDs), we study the effect of cavity quantum electrodynamics on the coupling of the microtoroid cavity. When with whispering gallery (WG) modes, the microtoroid cavity demonstrates high quality factor and small mode volume at visible wavelengths. Accordingly, fiber tapers allow QDs to adhere into the cavity and further permit the control of site-selected coupling. From the luminescence spectra, QDs are modulated effectively by cavity modes. Variable modulations are observed by changing QD coupling conditions. Therefore, based on experimental and theoretical research, strong and tunable Purcell enhancement can be realized by this system.The authors thank Jinming Cui and Chunhua Dong for their helpful discussion. This work was supported by the National Fundamental Research Program of China (No. 2006CB921900), the National Natural Science Foundation of China (Nos. 60537020 and 60621064), and the Knowledge Innovation Project of the Chinese Academy of Sciences.