ZnS, a kind of wide-band gap semiconductor, has attracted extensive attention in recent years due to its excellent photoelectric performance, which has broad application prospects in solar cells, photocatalysts and sensors. In this work, ZnS thin films are first deposited by radio-frequency (RF) magnetron sputtering, and then annealed at 600 ℃ and different sulfur pressures. The crystal structure, surface morphology, grain size, composition, transmittance and defects of ZnS thin films are analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, UV-vis transmission spectra, and positron annihilation Doppler broadening spectroscopy. The results show that the crystallinities of ZnS films can be improved by being annealed in sulfur atmosphere, and the optical band gaps of ZnS films after being annealed are in a range of 3.43–3.58 eV. When the sulfur pressure is higher than 0.49 atm, the sulfur interstitial atoms in the ZnS and the elemental sulfur on the surface reduce the transmittance of the film in the visible region. The Doppler broadening spectroscopy results also show that the defect concentrations of ZnS films decrease gradually from their surface layers to the inner layers, and the defects of ZnS films decrease with the increase of sulfur pressure. Meanwhile, the 3γ annihilation also proves that the interior of the film is relatively dense, and the open porosity of the film will increase due to vulcanization. Adsorbed sulfur occupies the position of sulfur vacancy defect in the crystal through internal diffusion, which leads the defect concentration to decrease and the film quality to be improved.