Abstract: ASM3-bio-P model is based on ASM3 with an additional module for phosphorus removal using modified processes from ASM2d. The model includes removal of organic, nitrogen and phosphorus. This study uses ASM3-bio-P model to set up a computer simulation system to model an actual A/A/O process in a municipal wastewater plant. Method of quickest descent is applied to calibrate model parameters. This enables us to model and predict effluent quality of municipal wastewater treatment plant in the indices of COD, TN, TP. The results show good correspondence between measured and simulated values. The feasibility of applying ASM3-bio-P model to simulate a WWTP in China is validated and an optimization method on solving calibration of model parameters is also proposed.