Abstract: To find out the reason of the discoloration and scaling-off of the polyurethane foam in aircraft fuel tank, detection of fuel oil microorganism was conducted, the discoloration area of the foam slag observed, thermal weight loss and FTIR tested.Resultsshowed that the number of microorganism in the oil tank was high above the standard. A large number of coccus, fungus and mildew were found attached to the surface of the foam, and many gnawed holes appeared at the attached positions under the action of microbial degradation. Thermo gravimetric results showed that foam degradation occurred. The failure of foam in the fuel tank was mainly caused by excessive microorganisms.