Abstract: The S-doped lignin-based carbon dots(L-SCDs) were prepared by hydrothermal method after acid hydrolysis of p-toluenesulfonic acid from alkali lignin. The morphology, structure and optical properties of L-SCDs were characterized by transmission electron microscopy(TEM), infrared spectroscopy(FT-IR), X-ray photoelectron spectroscopy(XPS), ultraviolet-visible absorption spectroscopy(UV-vis) and fluorescence spectroscopy(FL), and the response properties of L-SCDs in different pH environments were investigated. The results showed that L-SCDs was mainly composed of graphite-like carbon nuclei and luminescent units on their surfaces, and it had good dispersion with an average particle size of 2.19 nm. L-SCDs mainly contained three elements of carbon, oxygen and sulfur, and there were hydroxyl, carbonyl, carboxyl and other functional groups on their surface, indicating that alkali lignin reacted with p-toluenesulfonic acid and S element was successfully doped into L-SCDs. The study on the fluorescence changes of L-SCDs at different pH values showed that the fluorescence intensity of L-SCDs decreased significantly in alkaline environment, and the decrease was more obvious with the increase of pH value, which was mainly owing to the existence of carboxyl groups in the luminous unit of L-SCDs. The carboxylate was formed in alkaline environment, the luminous unit was destroyed, and the fluorescence intensity decreased. However, there was no significant change in acidic and neutral environment, indicating that L-SCDs had the characteristic of selective response to pH and could be used as a fluorescent probe to detect acid-alkalinity.