In order to understand the degradation characteristics of polylactic acid（PLA）square sand barriers,the PLA square sand barriers paved for 3.5years were investigated in different slope directions and at different slope positions in the U Lan Buh Desert.Changes in the mechanical properties of the PLA sand barrierswere studied by means of yarn tensile test.Changes in loss ratio were weighed by electronic balance andchanges in surface morphology of sand barriers were described by environmental scanning electron microscope（ESEM）.Results are as follows:Compared with the original PLA sand barriers,yarn breaking tenacities of four positions of the PLA sand barrier decreased differently with slope position ascending,and the order from low to highwas windward side,top side,leeward side and ground side.Loss ratios gradually increased from the bottom to thetop of the slope.The degradation of exposed section of the sand barrier was more serious than that of ground section.Blastation and abrasion by wind flow is the most important factor to the degradation of the PLA sand barrier, ultraviolet irradiation is in the second place,and microorganism in sandy soil has the minimal impact.