Short wave near-infrared (SWIR, 900–1700 nm) fluorescence imaging has attracted extensive research interest from scientists due to its high imaging quality. However, the variety of SWIR fluorescence imaging agents are quite limited and the corresponding quantum efficiency is relatively low. In this work, a novel conjugated polymer PDTSDTBT was reported, consisting of a donor unit with a tetrahedral Si (sp3) named DTS and an acceptor unit named DTBT with branched side chains. The design approach of endowing the donor–acceptor structure with the branched side chains successfully increase the fluorescence quantum efficiency. The polymer was prepared into nanoparticles by nanoprecipitation. The PDTSDTBT nanoparticles showed an absorption peak of 626 nm and fluorescence emission peak of 924 nm. The quantum efficiency of the nanoparticles is 0.53%, which is higher than that of nanotube fluorophores (0.4%). The nanoparticles also demonstrate a photothermal effect, the temperature of nanoparticles solution could reach 45 ℃ under excitation by 660 nm laser. Therefore, the PDTSDTBT nanoparticles is an excellent fluorescent imaging agent with potential photothermal applications.