Abstract: Disclosed herein is a carbon nanodot-polyacrylic acid composite hydrogel including a polyacrylic acid-based gel matrix having carboxyl groups, and a plurality of fluorescent carbon nanodots having amino groups on surfaces thereof. The fluorescent carbon nanodots are formed by subjecting polyethylenimine and hydrochloric acid to a hydrothermal reaction, and are distributed in the polyacrylic acid-based gel matrix. The amino groups of the fluorescent carbon nanodots are covalently bonded with the carboxyl groups of the polyacrylic acid-based gel matrix. Also disclosed herein are a method for preparing and a formulation for forming a carbon nanodot-polyacrylic acid composite hydrogel.