Abstract: The effect of iron oxide content in fly ash on the adsorption characteristics of polycarboxylate super-plasticizers with different structures was studied by the experiments of fluidity of paste,~potential, total organic carbon (TOC) and infrared spectra. The influence mechanism of iron oxides on the cement dispersion properties of polycar-boxylate superplasticizers was discussed and the improved method was put forward. It is found that iron oxide content in fly ash has a significant impact on the fluidity of cement paste mixed with polycarboxylate superplasticizers. When iron oxides are 18% of the total fly ash by mass, the corresponding slurry fluidity will be nearly lost; adding sodium sulfide can reduce the adsorption characteristics of iron oxides onto polycarboxylate superplasticizers, and the slurry fluidity is improved. Compared with common fly ash, high iron oxide fly ash has stronger adsorption characteristics onto polycarboxylate superplasticizers, with organic carbon content in the additive solution decreasing by one-third. Electrophoresis experiments indicate that the more the iron oxide content, the smaller the absolute value of~potential and the poorer the dispersion stability. Infrared spectra show that the adding of sodium sulfide reduces the adsorption characteristics of high iron oxide fly ash onto polycarboxylate superplasticizers.