Step response test methods are generally used to evaluate synchronous generator excitation system performance, but this method can not be implemented online. A method for evaluating the excitation system performance of the minimum variance control benchmark is proposed. Performance of the system under the action of the minimum variance controller output is considered as the upper bound of performance. The ratio of this output performance and actual output performance of the system is defined as the performance index. To avoid expanding the Diophantine equation, filtering and correlation analysis (FCOR) algorithm is introduced. The analysis results show that this method only requires synchronous generator output voltage data and a priori knowledge of the system dead time. Simulation results show that this method simplifies the calculation process, and evaluates the performance of excitation control system timely and accurately.