Determination of incremental oil is the key to polymer flooding performance evaluation and economic analysis. For commonly used polymer flooding methods to evaluate incremental oil, the production decline analysis method is the most widely used, however,when production rate changes greatly during the process of polymer flooding, the calculation of incremental oil using this method will result in a great error. This method is improved in this paper for eliminating the impact of extracts. Also, the analysis is conducted compared with the numerical simulation method, which has confirmed the reliability of the method. Calculation examples for north Buzachi polymer flooding test wells group show that, the improved production decline analysis method can be comprehensive and objective for evaluation of the effect of increasing oil of polymer flooding, which will provide scientific guidance for decision-making on polymer flooding.