The invention discloses a method for detecting the biological membrane ageing state in a sewage and wastewater processing system, and belongs to the technical field of water processing. The method includes the steps of firstly, conducting biological membrane sampling; secondly, conducting biological membrane preprocessing; thirdly, measuring the content of triphosadenine, adenosine diphosphate and adenosine monophosphate in a biological membrane; fourthly, calculating the adenylate energy charge of the biological membrane, and judging the ageing state of the biological membrane according to the numerical value of the adenylate energy charge, wherein the state is judged to be the ageing state if the numerical number is smaller than 0.6. The ageing state of the biological membrane can be judged directly through the adenylate energy charge, operation is easy and convenient, the result is visible, repeatability is high, the method is suitable for biological membranes of different sources, the efficiency of biological membrane method sewage processing operation monitoring can be remarkably improved, and normal running of a biological membrane method sewage processing system is ensured.