To clarify the influence of calcium hydroxide on the morphology of calcium sulfate hemihydrate hydrothermal products in the magnetized water system, scanning electron microscopy (SEM), conductivity analysis, and X-ray diffraction (XRD) were used to characterize the calcium sulfate hemihydrate samples. On this basis, the effect mechanism of calcium hydroxide on hydrothermal products of calcium sulfate hemihydrate crystal morphology was clarified. The results revealed that calcium hydroxide with different concentrations had an extremely significant regulatory effect on the morphology of hydrothermal products of calcium sulfate hemihydrate in the magnetized water system. With the increment of calcium hydroxide concentration, the morphology of hydrothermal products first changed from fibrous to plate-like, then to columnar. In this process, granular morphology was observed, which may be a mixture of calcium hydroxide and calcium sulfate dihydrate crystals. When the concentration of calcium hydroxide was 5.0×10-7 mol/L, hydrothermal products mainly existed as fibrous. With calcium hydroxide concentration increased to 5.0×10-5 mol/L, Ca2+ inhibited the dissolution of calcium sulfate dihydrate through the common ion effect, resulting in a diminishing of SO42- in the solution. Meanwhile, OH- and SO42- competitively adsorbed on the (002) crystal plane of calcium sulfate hemihydrate crystal in the form of chemical adsorption, which was not conducive to the precipitation of growth components on the crystal plane and led to the morphology transition from fibrous to plate-like. With the concentration of calcium hydroxide further expanded to 5.0×10-3 mol/L, excessive Ca2+ concentration in the solution was beneficial to the growth of hemihydrate calcium sulfate crystals along (200) and (110) crystal planes, while OH- restrained the growth of hemihydrate calcium sulfate crystals on (002) crystal plane. Beneath the combined influence of the above two factors, calcium sulfate hemihydrate transformed into columnar. This work provides references for the preparation of calcium sulfate hemihydrate hydrothermal products with different morphologies in magnetized water systems.