Abstract To deal with a huge amount of Cs-polluted soil produced from the first nuclear power plant in Fukushima (Japan), a mild hydrothermal conversion of Cs-polluted soil (metakaolin) into pollucite for immobilization of Cs in-situ was developed by simulating the geological evolution of pollucite in nature. The results showed that pollucite or pollucite analcime solid solution could be hydrothermally synthesized from Cs-polluted soil (metakaolin) at low temperature (200 °C). The X-ray Absorption Fine Structure (EXAFS) results showed that the polluted Cs of soil was immobilized into the structure of the synthesized pollucite or pollucite-analcime solid solution rather than physically adsorbed in the pores. The Near Infrared (NIR) and Thermal Gravity (TG) analyses revealed that analcime and pollucite could be distinguished effectively from their water molecular contents. The leaching tests results showed that Cs concentrations leached the synthesized pollucite and solid solution immobilization were 1.5 × 10 −3 g/(m 2 ·day) and 5.17 × 10 −5 g/(m 2 ·day) similar to that of ceramic immobilization (1.65 × 10 −4 g/(m 2 ·day)). As such, the hydrothermal conversion of Cs-polluted soil into pollucite for immobilization of Cs in situ might provide a new technology for the huge amount of radioactive polluted soil treatment.