Abstract: The phosphorus removal by the biological nutrient removal system depends on the exclusion of excess sludge，which requires short SRT micro-organisms；while the nitrifying bacteria is autotrophic，its growth requires a longer time. At low temperature (≤10℃)，the nitrification sludge age is generally 15－20 d，and PAOs sludge age is 4－5d，this enormous gap resulted in the difficult to achieve simultaneously nitrogen and phosphorus removal at low temperature with the A²/O process. Thus，the effect of nitrogen and phosphorus removal using improved inverted A²/O process is studied，through which the COD removal rate of higher than 85%，NH₄⁺-N removal of 85% and T-P removal efficiency of 80 % were achieved at low temperature conditions. With this improved inverted A²/O process，the effluent can reach the national emission standards for secondary water.