Composite　insulators　are　widely　used　in　transmission　lines,　especially　in　overhead　lines　due　to　their　excellent　surface　hydrophobicity　and　good　performance　against　pollution　flashover.　Some　composite　insulators　are　used　in　large-span　long-distance　transmission　lines　through　plateaus,　mountains　or　other　high　altitude　areas.　Due　to　thin　atmosphere,　ultraviolet　(UV)　radiation　in　high　altitude　areas　is　extremely　intense.　High-intensity　UV　radiation　will　destroy　outer　surface　hydrophobicity　of　composite　insulators,　resulting　in　significantly　increasing　pollution　accident　rate　of　high　altitude　transmission　lines.　In　this　paper,　4000　hour　UVA,　UVB　and　UVC　radiation　experiments,　and　scanning　electron　microscope　(SEM)　and　energy　dispersive　spectrometer　(EDS)　detections　were　performed,　effects　of　different　wavelength　UV　on　hydrophobicity　of　composite　insulators　were　obtained.　It　was　found　out　that　UVC　ultraviolet　was　the　main　factor　causing　loss　of　surface　hydrophobicity　of　composite　insulators.　Finally,　concrete　influence　mechanism　was　put　forward.　©　2018,　Power　System　Technology　Press.　All　right　reserved.