Plant cell walls (CWs) with complex macromolecular structures can surround and protect cells from a variety of harsh environmental conditions such as pathogens, herbivores, and trace metals. Here, a novel strategy for in situ imaging of plant cell walls was developed to evaluate heavy metal pollution via thiolated full-color emissive carbon-dots (F-CDs) targeting Pb(ii)-adsorbed onion cell walls. The thiolated F-CDs with excellent optical properties from red light to blue light were synthesized through a facile electrochemical approach using new precursors of luminol and l-tryptophan and further modified with l-cysteine. Based on a strong covalent interaction of Pb(ii) and thiolated F-CDs, we achieved in situ fluorescence imaging for the Pb(ii) adsorbed on CWs, which showed enhanced red, blue and green multi-color fluorescence (FL) on CWs with increased Pb(ii)-ion content. In contrast, multi-color fluorescence on cytoplasm diminished, attributed to F-CDs targeting and accumulating on the cytoskeleton which thus limited F-CD diffusion into protoplasm. Therefore, in situ fluorescent images for CWs can demonstrate heavy metal contamination degrees in plant cells. This facile and undamaging protocol will be beneficial for investigating heavy metal migration into the protoplast and fast evaluation of food quality and safety.