Sucrose was added to the hydrothermal decomposition reaction of calcium oxide and K-feldspar to improve the adsorption performance of the hydrothermal product tobermorite. The effects of the sucrose concentrations on the decomposition rates of K-feldspar were studied based on the extraction rates of potassium oxide, and the influence mechanism of sucrose on the phase change of K-feldspar during the hydrothermal decomposition was elucidated. It was shown that when n(Su)/n(Ca)=0.1:1, the extraction rate of potassium oxide was 83.83%, and the decomposition of K-feldspar was inhibited with the increase of the sucrose concentration. The increase in the viscosity of the reaction system and the formation of calcium complexes during the hydrothermal process were the main reasons for the inhibition of the decomposition of K-feldspar. Moreover, the enhancement mechanism of the methylene blue adsorption capacity of the sucrose-modified tobermorite was explained from the perspective of the microstructure.