Equivalent circuit method is a principal one to analyze the active frequency selective surface (FSS). Extracting its lumped parameters is the key to the equivalent circuit method. We have constructed the transfer function based on the traditional equivalent circuit method and the transmission line theory. A matrix equation composed of lumped parameters is set up utilizing the relationship between the equivalent impedance and transmission peak. The equivalent lumped parameters are solved by the least square method, and the FSS frequency response curves are obtained from the transfer function. Compared with the full wave analysis method, the calculated results are in good agreement with that of simulation. Such results verify the accuracy and reliability of the method presented in this paper, and provide a theoretical reference to active FSS analysis using the equivalent circuit method.