The structures of six possible native point defects (I and In vacancies, I and In antisites, I and In interstitials) that maybe exist in the orthorhombic indium iodide (InI) crystal are optimized and investigated by the first-principles calculations based on density functional theory. The levels of difficulty in forming defects in their growth processes are obtained by calculating the defect energy levels; the position of each kind of energy level of native point defect and its effect on carrier transport are analyzed via calculating the density of states. The results show that the dominant low-energy defect of In interstitial induces a recombination center and a deep hole trap: the former shortens the lifetime of the minority carriers and the latter captures the holes from the valence band, thereby reducing the mobility-lifetime product of the hole. The calculation results provide a theoretical guidance for improving the mobility-lifetime product of carriers in InI crystal and also are helpful in obtaining the excellent materials for detecting the nuclear radiation of InI crystal.