In this paper, calcium sulfate hemihydrate whiskers were prepared by autoclaving using phosphogypsum as raw material. Digital microscope, X-ray diffraction (XRD) and X-ray photoelectron spectrometer (XPS) were used to investigate the microscopic morphology, composition and crystal surface development of calcium sulfate hemihydrate whiskers under the effect of Mg2+. Using molecular dynamics technology to simulate the interaction energy and radial distribution function of Mg2+ in the crystal surface of the calcium sulfate crystal. The effect of Mg2+ on the crystallization habits of calcium sulfate crystals was explored by combining the two techniques. The results show that the interaction energies of Mg2+ and different crystal faces of calcium sulfate hemihydrate crystals from smallest to largest are listed below: ΔE(200)<ΔE(002)<ΔE(400)<ΔE(020)<ΔE(204), which illustrates that Mg2+ is mainly adsorbed on the (200), (400) and (002) crystal faces of calcium sulfate hemihydrate crystals. The interaction distance between Mg2+ and SO2-4 is smaller than that between Ca2+ and SO2-4. There is an obvious electronic effect between Mg and S, and the electronic effect is enhanced with the increase of magnesium sulfate content. The position of XRD diffraction peak is more obvious to the right with the increase of magnesium sulfate. All indicate that a small amount of Mg2+is embedded in the crystal lattice of calcium sulfate hemihydrate. At 0.15% (mass fraction, the same as below) of magnesium sulfate content, the growth of c-axis direction was preferentially promoted, resulting in slender needle-like calcium sulfate hemihydrate whiskers with an average length of 47.29 μm and an average L/D ratio of 36.07. At 0.30% of magnesium sulfate content, the growth of c-axis and radial direction was inhibited, resulting in slender needle-like calcium sulfate hemihydrate whiskers with an average length of 39.72 μm and an average L/D ratio of 28.91.