An inverted A²／O(anoxic／anaerobic／aerobic-pre-anoxic)一MBR process was applied in Hefei Tangxihe reclaimed water treatment plant to remove nutrients from low C／N ratio municipal sewerage. The variances of organics and nitrogen in each operational unit with and without addition of extrnal carbon source were studied. Results showed that the TN removal efficiency was only 52.3％ without addition of external carbon source. The conclusion that sodium acetate had the best effect and the higllest denitrification rate can be obtained by comparing the effects of three different extra carbon sources-sodium acetate, methanol and acetic acid. Therefore, sodium acetate was added as external carbon source with a theoretical amount of 50 mgCOD／L in influent and the TN removal efficiency was enhanced significantly to 74.5％. Besides，the material flow of COD arld TN were systematically analyzed after the soldium acetate was added. The results indicated that the anoxic tank and anerobic tank were the main process units for COD removal, while the aerobic tank enhanced the effect of COD removal efficency. The removal rate of TN in system significantly increased 26.5％ after the dosing of external carbon.