Hydrogels have great applications in tissue engineering and drug delivery. Recently, there have been intense interests in developments and applications of nanocomposite hydrogels by incorporating nanomaterials into polymer matrix, which endows the hydrogels with new functionalities. Here, we report on the first carbon dots (CDs) doped hydrogel waveguide for selective, on-site detection of heavy metal ions in aqueous solutions. The CDs-doped hydrogel waveguide exhibits efficient light confinement in water due to the refractive index contrast. The smooth waveguide surfaces lead to low light scattering loss. Real-time spectra measurement of the CDs-doped hydrogel waveguide with a compact interrogation setup demonstrates that the novel design can be used as a portable, robust sensing platform for on-site analysis and assessment of heavy metal ions.