Fluorescent conjugated polyelectrolytes (CPEs) with a large extinction coefficient, high quantum efficiency, good photostability, and efficient intramolecular/intermolecular exciton migration have recently received great research attention as a new generation of fluorescent probes. Herein, the unique properties of fluorescent conjugated polyelectrolyte and mesoporous silica nanoparticles (MSNs) are combined to prepare a pH-sensitive drug delivery and cellular imaging system using a simple and more general method. To the best of our knowledge, this is the first proof-of-concept investigation on the use of fluorescent conjugated polyelectrolyte as both the capping agent of MSNs and the cellular imaging probe in parallel. Moreover, the bifunctional nanomaterial exhibits low cytotoxicity and good biocompatibility demonstrated by a cytotoxicity assay against HepG2 cells and a hemolysis assay against human red-blood cells. It is a promising candidate for simultaneous diagnostics and therapeutics and can also provide a general strategy to develop novel multifunctional nanomedical systems.