In this paper, the factors related to the morphology of calcium sulfate whisker (CSW) are discussed. Most attention was devoted to the influence of co‐crystallized phosphorus in phosphogypsum—existing in the form of CaHPO4 · 2H2O—on the crystal growth, which was ignored in the preparation of CSW from phosphogypsum in previous research. Methods including X‐ray powder diffraction and scanning electron microscopy were performed to characterize the resultant crystals. The CSWs with uniform morphology were prepared by hydrothermal synthesis with commercial CaSO4 · 2H2O. However, the productions from the pretreatment phosphogypsum by hydrothermal synthesis appeared as irregular crystals with a small length to width ratio, and whisk‐broom‐like crystals were common. The influence of CaHPO4 · 2H2O on the growth of CSW has also been studied. The results showed that CaHPO4 · 2H2O can promote the formation of needle‐like crystals and whisk‐broom‐like crystals at hydration condition. In order to improve the morphology of CSW in a CaSO4 · 2H2O‐CaHPO4 · 2H2O system, substantive condition experiments were carried out, and it finally revealed that Fe3+ was useful for the well‐developed CSW.