With its excellent workability, corrosion resistance, chemical resistance, UV aging and environmental stress cracking resistance, etc., Polyethylene is a major cable material for bridge girder. But early cracking is of prevalence of all bridges sheathed by polyethylene cable; rope has to be replaced or repaired far less than the actual design life of 50 years.As the material aging,the dgree of crosslinking goes up,which may cause stess concentration and the crack through the girder.If the plyehtylene is pro-crosslinked before we use them,the stess concentration can be prevented and their life can be increased.But as polyethylene aging,the dgree of crosslinking goes up too.We must find out at what dgree the material shows fantastic UV aging resistance.What’s more,the thickness of crosslinked layer may effect on aging of polyethylene.Crosslinked layer creeps slower while the un-crosslinked part creeps faster,which may cause complex mechanism of aging.Research on the effect of the thickness of crosslinked layer to the aging of polyethelene needs studying. Task Force designed and manufactured a set of constant temperature and humidity, stress-photo aging device to simulate the aging of sheath material. Through the methods of the surface morphology observation, deformation measurement, DSC analysis, surface ATR-FTIR analysis and the mechanical properties testing to study the cracking behavior of polyethylene of different dgrees of crosslinking and different thickness of corsslinked layer during photo-oxidation.