The exposure test of media density polythene(MDPE) was carried out for different time periods up to one year in Xisha(Paracel) Islands.The changes of appearance,morphology,mechanical properties and chemical structures were monitored by spectrophotometer,glossmeter,microscope,computer-controlled tensile testing,hardness measurements and Fourier Transform Infrared(FTIR) spectroscopy.The results showed that during the aging period,the color abbreviation and hardness increased,while the gloss,the retention of tensile strength and that of elongation at break decreased.The aging evaluation parameter Z was postulated,which can present the common of those tested properties well,which showed a stepwise increasing tendency of aging rate with the extending of exposure time.