Wide band gap γ-CuI is a p-type transparent semiconductor with excellent optoelectronic and thermoelectric property, which has recently attracted worldwide attention. However, as an emerging material, its luminescence mechanism that is impacted by defects is rarely reported and remains obscure, limiting its further applications. In this work, Cl-doped CuI film was prepared by gas-phase reaction method. Using cathodoluminescence spectroscopy, effects of Cl doping on the surface morphology and cathodoluminescence property of CuI films were investigated in detail, and main defects of Cl presence in CuI films were explored by combining first-principle calculations, revealing relationship between structure and luminescent property of Cl-doped CuI films. These data showed Cl-doped region had a smoother surface than that of the undoped region with granular morphology, which clearly demonstrated that Cl dopant altered surface structure of the undoped region. Compared with the undoped region, the Cl dopant induced doubled fluorescence signal of band-edge emission at 410 nm, but reduced the defect peak at 720 nm, indicating that a small amount of Cl dopant brought a great luminescent improvement to CuI. The formation energy calculations of various crystal defects suggest that Cl can inhibit the formation of deep-level defects such as I vacancy in CuI and reduce the probability of non-radiative transition of excitons, which is consistent with the cathodoluminescence results. The full width at half maximum of the band-edge luminescence peak of Cl-doped CuI film is as small as 7 nm, showing extremely high luminescence monochromaticity. Therefore, the present findings deepen our understanding on how halogen doping boosts the luminescence performance of CuI-based materials.