Abstract: Nuclear energy, as a clean energy source, is accompanied by the generation of nuclear waste during its development and utilization. Adsorption is a primary method for treating radioactive wastewater. Compared to other adsorbents, fly ash-based geopolymers offer advantages such as being green, low-carbon, cost-effective, and adsorption-friendly. Although there have been many studies on the adsorption characteristics of heavy metal elements in wastewater by fly ash-based geopolymers, the adsorption characteristics of radioactive elements remain unclear. In this study, fly ash was used as the raw material to synthesize fly ash-based geopolymers through different liquid-to-solid ratios. Adsorption experiments were conducted to evaluate the adsorption performance of these geopolymers for simulated radionuclides Cs (Ⅰ),Co(Ⅱ), and Eu(Ⅲ),as well as their competitive adsorption behavior. The results showed that the adsorption of the three radionuclides gradually increased with the extension of adsorption time, reaching equilibrium after approximately 50 hours. Under different liquid-to-solid ratios, the adsorption performance of Eu(Ⅲ) and Cs (Ⅰ)showed significant differences, with the geopolymer at a liquid-to-solid ratio of 0.7 exhibiting the best adsorption capacity for all three radionuclides. Eu(Ⅲ)maintained a high adsorption rate over a wide concentration range, indicating strong adsorption behavior. Additionally, the study found competitive adsorption behavior between the radionuclides. Eu(Ⅲ)showed a strong competitive adsorption ability over Cs (Ⅰ) and Co(Ⅱ),exhibiting the highest adsorption capacity. When the three radionuclides coexisted, the adsorption rate of Eu(Ⅲ)remained high, while that of Cs (Ⅰ) and Co(Ⅱ)decreased significantly. Compared to conventional adsorbents, fly ash-based geopolymers not only possess good adsorption performance but also offer low cost and environmental benefits, achieving a waste-to-waste solution. The findings provide important data support and theoretical basis for the treatment of radioactive wastewater using geopolymer adsorption, with significant application prospects.