<p>Optimizing the properties of PbS thin films is significant to fabricate high-performance PbS-based optoelectronic devices. The properties of PbS thin films are greatly influenced by the Pb/S molar ratio in the raw materials and annealing atmosphere. In this study, the effects of Pb/S molar ratio (1:1, 1:2, and 1:3) in the raw materials and annealing atmosphere (air, N₂ + O₂, and O₂) on the structure, morphology, and optical properties of PbS thin films were systematically investigated. As the Pb/S molar ratio increases, the (200) preferential orientation of PbS thin films becomes pronounced, which further changes the morphology of PbS and film thickness. Meanwhile, the crystallinity decreases and microstrain of PbS thin films increases. PbS thin film prepared with a Pb/S molar ratio of 1:1 exhibits the best crystallinity, the biggest crystallite size, the lowest microstrain, and the narrowest band gap. Annealing in oxygen-containing atmosphere promotes the formation of PbO·PbSO₄ phase with elongated plate-like morphology, and the amount of the oxidation phase increases with the increase of oxygen content in the annealing atmosphere. The increased amount of oxidation phase is beneficial to enhance the light absorption of PbS thin films. After being annealed in different atmospheres, the band gap of the PbS thin film prepared with a Pb/S ratio of 1:1 notably decreases from 2.41 to the range of 1.68–1.69&#xa0;eV.</p>