Recently, solar photovoltaic (PV) systems have been helping to relieve the global energy crisis. However, the service lifetime of PV systems may be shorter than the designed time. The backsheet, which is usually made of polyethylene terephthalate (PET), influences the long-term reliability of PV systems. When a PV system is located in a coastal area, the backsheet usually suffers from various problems such as partial discharge (PD) activity induced by the leakage current caused by potential-induced degradation (PID) because of exposure to salt-mist. Here we used atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman spectroscopy, differential scanning calorimetry (DSC) and phase-resolved partial discharge (PRPD) to explore the PD activity of PET backsheets caused by PID after salt-mist exposure. The analyses revealed that the surface roughness of the PET backsheet increased with the salt-mist duration. Raman spectroscopy indicated that some chemical groups changed after PD activity and salt-mist exposure. The degree of crystallinity increased under the condition of PD activity with increasing salt-mist exposure time. The result of PRPD indicated that PD activity depended salt-mist exposure conditions. From these results, we found that the erosion rate of PET backsheets was influenced by salt-mist exposure and PD activity induced by the leakage current caused by PID. These findings provide guidance to improve the durability of insulating backsheets of PV systems.