It has very important application value to investigate the damage performance of PbS detector irradiated by mid-infrared laser. In this paper, the experiment research on damage in PbS detector irradiated by 2.79 μm mid-infrared laser is carried out. Furthermore, the theoretical model of PbS detector irradiated by 2.79 μm laser is developed and a numerical simulation is performed to calculate temperature distribution in PbS detector using finite element method, and the relationship between laser parameters and damage effect is also studied. The simulation and experimental results indicate that the damage mechanism of PbS detector irradiated by 2.79 μm mid-infrared laser is mainly melting damage, and the melting damage threshold is calculated to be 13.03 J/cm2. Before the temperature reaches the melting point of PbS, the thermal decomposition reaction of PbS begins, and then separates out PbO which is the yellow precipitate. It is shown that pulse repetition frequency and the number of pulses affect damage considerably, and the accumulation of multi-shot laser induced damage in PbS detector is obvious. The theoretical analysis is in agreement with the initial damage morphology of PbS surface.