Chlorinated organic materials have drawn much attention and have been applied in various fields due to their intriguing properties such as easy accessibility with low cost, high capability to hold electron density, and “heavy-atom effect”. In this work, a kind of highly fluorescent chlorinated semiconducting polymer dot (PFDPClBT) is developed for cell imaging by using an amphiphilic polymer poly(styrene-co-maleic anhydride) as the co-encapsulation matrix. The PFDPClBT dots show a small particle size of about 50 nm, a large Stokes shift of 125 nm, quite high fluorescence quantum yields (QYs) of 20.3% (PFDPBT: 8.5%), and increased reactive oxygen species generation efficiency. Since the chlorinated Pdots are brighter than their non-chlorinated counterparts and can label the target cell effectively with low cytotoxicity, they are promising fluorescent probes in bioimaging applications.