Abstract: The method for performance evaluation and simulation of wastewater treatment plants to support process retrofitting was introduced in this study, with a case study to illustrate the analysis procedure. By historical data tracking and monitoring campaign along the process, tempo spatial variation of the pollutants was understood for estimating the performance of each functional zone in the process. By assessing the biological activity of activated sludge, the reaction rate and potential of nitrification, denitrification and phosphorus release were acquired and analyzed. Moreover, process modeling and scenario analysis were applied to quantify the key factors of removal efficiency of the process. As a result, suggestions on the upgrade design and operation optimization were proposed after the analysis, proving the feasibility of the integral method to assist the process retrofitting.