In this paper, the use of biocompatible carbon dots (C-Dots) as caps on the surface of mesoporous silica nanoparticles (MSPs) for the design of intelligent on demand molecular delivery and cell imaging system is described. These C-Dots-attached MSPs exhibited low cytotoxicity toward the cells and strong luminescence both in vitro and in vivo. A further loading of anticancer drug (DOX) endowed the fluorescent material with therapeutic functions. It was found that changing the pH to mildly acidic condition at physiological temperature caused the dissociation of the C-Dots@MSPs complex and release of a large number of DOX from the nanospheres. Moreover, the DOX-loaded nanocomposites exhibited a remarkably enhanced efficiency in killing cancer cells. The endocytosis and the efficient drug release properties of the system were confirmed by luminescence microscopy. Overall, we believe that the well designed C-Dots@MSPs nanocomposites are promising for a simultaneous bioimaging and drug delivery system, which show more potential for clinical application.