InSeI single crystals were synthesized by the chemical vapor transport (CVT) method. The crystal is yellow needle-shaped and brittle. X-ray diffraction results at room temperature show the tetragonal system of InSeI, with lattice parameters of a=b=1.864 3(5) nm, c=1.012 0(3) nm, V=3.517 2 nm3, and space group is I41/a. The ultraviolet-visible absorption spectrum, photoluminescence spectrum results show that InSeI has a 2.48 eV band gap, under the excitation of a certain band of light, InSeI single crystal has a wide emission peak at about 600 nm, which indicates that the luminescence mode of the crystal is defect state luminescence. The dielectric temperature spectrum indicates that a phase transition happened in the tetragonal structure of InSeI crystals at 440 K.