Abstract: In order to investigate the mechanism of snail trail defects of photovoltaic modules during outdoor exposed process, a comprehensive analysis was carried out including the degradation of electrical properties, the SEM of the snail trail, the element of Ag finger and EVA encapsulant with snail strail. As a result, the climate of hot-humid and technology of photovoltic modules are the main causes of snail trail. It will be effective to avoid snail trail by improving the technology of PV modules and choosing PV modules depending on adaptation to local conditions.