PEDOT∶PSS (poly 3,4-ethylenedioxythiophene-polystyrene sulfonate) film is widely used due to its good electrical conductivity, light transmittance, mechanical flexibility and solution processability. Improving the electrical conductivity and photoelectric stability of PEDOT materials is of great significance to its device applications. In this article, highly conductive PEDOT∶PSS films were obtained by spin-coating method after post-treatment by sulfuric acid. The films were placed in air, oxygen, nitrogen and under sun illumination for 30 d to study their photo-electrical stability and aging mechanisms. It indicates that: sunlight is the main factor affecting the stability of photo-electrical properties of thin films, and sulfuric acid treatment removes the excessive PSS component effectively and therefore improves the stability of PEDOT∶PSS films. By comparing the absorption spectrum and photoelectron spectroscopy (XPS) of the film before and after aging, it is found that, the film underwent photo-oxidation degradation during the aging process, which directly affects its photoelectrical stability.