Carbon dots (CDs) are a new type of fluorescent carbon nanomaterials with highly carbonized cores and surface groups. The outstanding properties of CDs, such as excellent fluorescence properties, ultrasmall size, abundant functional groups, and high biocompatibility, make CDs-based nanoplatforms widely used in biomedical fields. A summary of the properties of CDs can increase the understanding and facilitate the applications of CDs-based nanoplatforms. Based on this, the vital physicochemical properties of CDs are innovatively summarized in this review, including fluorescence, ultrasmall size, tumor targeting, light absorption, hydrophilicity/hydrophobicity, and catalytic capacity. To further broaden the biomedical applications of CDs, various CDs-based nanoplatforms are summarized and the CDs complexes are creatively classified in terms of “host/guest” molecules. At the same time, synthesis, improvements, and recent advances in CDs such as bioimaging, tumor theranostics, biomacromolecules delivery, biosensing, antioxidation, and antibiosis are also discussed in detail. The challenges in further understanding and applying CDs are rationally discussed.