Abstract: The relative molecular weight was an important factor affecting the performance of polymers. Four types of sodium lignosulfonate-graphene quantum dots(SL/GQDs) were prepared using sodium lignosulfonate with different relative molecular weights and citric acid as raw materials. The prepared SL/GQDs solutions were labeled as SL/GQDs1-SL/GQDs4, corresponding to relative molecular weights of undefined, 5 000, 10 000, and 50 000, respectively. Optical properties, structures, and morphologies of the four composite quantum dots were compared and analyzed using UV-visible spectroscopy, infrared spectroscopy, transmission electron microscopy, and fluorescence spectroscopy, respectively. The results showed that the surface of SL/GQDs was rich in hydrophilic groups such as carbonyl and hydroxyl groups. The average particle sizes of SL/GQDs1-SL/GQDs4 were 1.34, 1.78, 2.13, and 2.26 nm, respectively. The composite quantum dot SL/GQDs4 with a relative molecular weight of 50 000 exhibited the best overall performance and the strongest photo-induced luminescence effect. Therefore, SL/GQDs4 was selected for the detection of metal ions. It was found that SL/GQDs4 showed good selectivity and sensitivity towards different metal ions, and Fe³⁺ could still be detected in the coexistence system of different metal ions, which could serve as a fluorescent probe for Fe³⁺ detection. Furthermore, the addition of Fe³⁺ resulted in fluorescence quenching effect in SL/GQDs4. When Fe³⁺ concentrations were in the range of 10-400 μmol/L, there was a good negative correlation between Fe³⁺ concentration and fluorescence intensity(F/F₀). The Stern-Volmer linear regression equation obtained by data fitting was F/F₀=0.893 66-0.001 19C(Fe³⁺), with R² of 0.98.