Stability under light is critical for lead sulfide quantum dots (PbS QDs) in solar cell applications. To improve the stability of PbS QDs solar cells, the influence of the light illumination on the performance of the as-prepared PbS QDs solar cells was carefully investigated. Combined with X-ray and ultraviolet photoelectron spectroscopies, it was revealed that the 1,2-ethanedithiol (EDT) ligands of the hole transport p-type PbS QDs reacted with the ligands of the PbS QDs active layer under light illumination. The reaction not only undermines the n-type characteristics of the active layer, but also increases the number of defects, leading to a serious deterioration in device performance. An interface layer was introduced to block the EDT penetration to avoid this issue, significantly improving the stability of the device under light irradiation. Moreover, the separation of carriers was also enhanced due to the better matching the energy level of the interface layer. The device with an interface layer yielded a power conversion efficiency of 12.55% and sustained to a long time light illumination.