PbS films grown on quartz substrates by the chemical bath deposition method were annealed in an O₂ atmosphere to investigate the role of oxygen in the sensitization process at different annealing temperatures. The average grain size of the PbS films gradually increased as the annealing temperature increased from 400 °C to 700 °C. At an annealing temperature of 650 °C, the photoresponsivity and detectivity reached 1.67 A W⁻¹ and 1.22 × 10¹⁰ cm Hz^1/2 W⁻¹, respectively. The role of oxides in the sensitization process was analyzed in combination with X-ray diffraction and scanning electron microscopy results, and a three-dimensional network model of the sensitization mechanism of PbS films was proposed. During the annealing process, O functioned as a p-type impurity, forming p⁺-type PbS layers with high hole concentrations on the surface and between the PbS grains. As annealing proceeds, the p⁺-type PbS layers at the grain boundaries interconnect to form a three-dimensional network structure of hole transport channels, while the unoxidized p-type PbS layers act as electron transport channels. Under bias, photogenerated electron–hole pairs were efficiently separated by the formed p⁺-p charge separation junction, thereby reducing electron–hole recombination and facilitating a higher infrared response.