The photosensitive properties of PbS films can determine the performance of PbS detectors. The PbS films are the core and foundation of PbS infrared detectors and are also the research direction for the modification and improvement of PbS infrared detectors all the time. The reaction mechanisms of three processes including high temperature method, Kodak method and hydrazine method in the formation of PbS films are analyzed. The results show that the photosensitive films are all related to the formation of PbSO4. The X-ray diffraction test result shows that there are PbSO4 in PbS poly-films. So, it suggests that the formation of the PbS solid solution containing low concentration PbSO4 is one of the reasons for PbS films to have photosensitive properties.