Abstract: ¹³⁷Cs, with high radioactivity and heat generation, is one of the main fission products in spent fuel. The stable solidification of ¹³⁷Cs is beneficial to ensure the safety of its long-term disposal. The adsorption behavior, solidification properties and mechanism for Cs by allophane were studied in this paper. By varying the initial concentration and pH of the solution, the adsorption percentage of Cs is obtained more than 90% and the maximum adsorption capacity reaches 11.1 mg/g under the condition of pH=5. The adsorption mechanism was governed by the joint action of single-layer adsorption and multi-layer adsorption. The solidified bodies were prepared by pressing and sintering. The effect of sintering temperature and mixing ratios on the properties of the sintered product was studied. Experiments and density functional theory (DFT) calculations show that the crystalline phase of solidified product is formed at 1 200 ℃, the volume reduction ratio is over 55%, and the Cs immobilization ratio is nearly 100%. When the content of Cs in the solidified bodies is 1%, the K atom could substitute Cs atom through isomorphism and subsequent forming CsxK1-xAlSi₂O₆ crystal structure at high temperature. When the content of Cs reaches 10%, the main crystal phase is determined as pollucite. Cs atom is immobilized by forming ionic bonds and covalent bonds in the pollucite crystal structure with the bond energy of 5.85 eV.