Degradation of poly (butylene succinate-co-terephthalate) (PBST) mulch film in the vast cotton field in the dry and windy plain has been studied. The films within 3 months' use which were above the soil and far away from the plant were selected collected as they were under closely identical outdoor degradation conditions. In macro mechanical properties, the quick decay of the ductility was the most obvious change of the mulch film in outdoor use. The elongation at break in transverse direction (TD) decreased faster than that in machine direction (MD). From micro view, chemical groups are almost retained within 3 months, while three kinds of changes in the polymer chain have been observed: (a) most additives are lost after 1 month's use; (b) chain slightly hydrolysis, of which far less than 1 break per chain occurs at the end of the 3rd month; (c) chain crosslinks, of which there are ∼1.3, 4.1 and 5.2 crosslink points per chain in average after 1, 2 and 3 months' use. The chain crosslink of the PBST is considered to be the main reason for the big decrease of the mechanical property, especially the ductility of the film, and then the main reason for the large-scale breakage of the film during 1∼2 months in Xinjiang. UV screening agent which has a strong adhesion to the polymer should be considered to solve the mulch film problem in the dry and windy plain. •Accurate ductility decay of the muddy mulch film is got.•Most additives are lost at the time scale of ∼1 month.•Chains hydrolyze slightly, far less than 1 break per chain at the 3rd month.•Number of crosslink are ∼1.3, 4.1 and 5.2 points per chain after 1, 2 and 3 months.•Crosslink is the crucial factor for mulch film problem.