Abstract: Objective The hydrothermal method has attracted attention in recent years as an effective method for producing inorganic single crystals. The liquid-solid ratio emerges as a key parameter influencing the growth of gypsum crystals at hydrothermal conditions, with direct implications for product quality, energy consumption, and production efficiency. Method Herein, sintered flue gas desulphurization gypsum is adopted as raw materials to explore the effect of liquid-solid ratio on the preparation of columnar gypsum crystals at hydrothermal conditions. Result The raw materials consist of calcium sulfate dihydrate with various morphologies and diameters, including prismatic, lamellar, and column, as well as agglomerated silicate phases. However, after hydrothermal treatment, these desulfurization gypsum particles with different sizes can grow into columnar single crystal hemihydrate gypsum with controllable morphology. With increase in liquid-solid ratio, the compactness of crystal surface increases, pore defects disappear, and the length-diameter ratio is shortened gradually. ConclusionIn order to produce densely compact, short columnar hemihydrate gypsum with an aspect ratio of approximately 2:1, the optimal liquid-solid ratio is 10:1.These findings offer substantial insights into regulating the structure and morphology of desulfurization gypsum.