The phenomenon of denitrifying can be found under the condition of anaerobic/anoxic(A/A) in the biological phosphorus removal. It is found through the test that: The ability of denitrifying phosphorus removal in the system of A/A-SBR is lower than the system of A/AO-SBR, and along with the time increasing in the system of A/A-SBR, the ability of phosphorus removal would weaken, Ys will also decrease gradually from initial 0.22 gMLSS/gCOD.d to zero or even minus. The result of test indicates that the key to make the system oprerate steadily is increasing the latter aerobic time. However,the longer aerobic time will result in the accumulation of NO_3-N, and restrain the system of A/AO-SBR from removing phosphorus. It is also found in the test that the latter aeration for 0.5 h would ensure not only steady operation, but also good effect of phosphorus removal in the system of A/AO-SBR.