Alpha high‐strength hemihydrate gypsum (CaSO4·0.5H2O, α‐HH) with a large‐size columnar crystal (average size: 135.31 µm, maximum size: 500 µm) is successfully prepared via a NaCl salt bath method using salt gypsum as a raw material. The effects of preparation condition (salt solution concentration, salt bath time) on the crystalline phase and crystal grain size of the α‐HH and on the strengths of the cement bulks from α‐HH are investigated. Furthermore, to explore the formation mechanism of the large‐size columnar crystal of the α‐HH, the samples obtained by varying the salt bath time of the reaction system are characterized with field‐emission scanning electron microscopy, X‐ray diffraction, and laser particle size analysis. Based on the characterization results, a formation mechanism of the large‐sized α‐HH is proposed. That is, NaCl self‐diffusion creates a local high‐concentration salt solution environment, which is conducive to the recombination of small‐size HH crystals formed by dihydrate gypsum dissolution and HH recrystallization to grow into large‐sized α‐HH crystals.