The deep sea floor is recognised as one of the most important final destinations for plastic debris. It is not clear whether the plastic debris in the deep sea could be degraded. Likewise, little is known about how long plastics might last at the deep sea floor. A total of 103 plastic debris were recovered using the manned submersible “Shenhaiyongshi” on the deep sea floor (746–3997 m) of the South China Sea (SCS). We found that abundant corrosion structures were present on the surface of poly-ethylene (PE), which was the dominant type of plastic sample (80 %). The rod-like, filamentous and peanut-like morphologies of the corrosion structures are well in line with those of microorganisms, suggesting that they were derived from biodegradation. The calculation of volume loss of corroded PE showed that about 1.08–13.72 % PE were degraded. Assuming that the most degraded plastic reached the deep sea floor 40 years ago, these plastics will require about 292 years to be totally degraded. Our results provide unique insights into the fate of deep sea plastics and answer the unsolved question about how long plastics may persist in deep sea.