This paper presents a system modeling method for an multi-stage operational amplifier circuit based on the step response experiment. By taking an step voltage signal as the input, the measure data are collected for estimating the system parameters. By defining and minimizing an output error criterion function, a Newton iterative estimation algorithm is derived. In order to show the accuracy of the estimated circuit model, we use a proportional integral controller and construct a closed-loop operational amplifier circuit in terms of the estimated circuit model. Moreover, we take a step experiment to test the tracking performance of the operational amplifier circuit. The simulation results indicate that the proposed method can generate the parameter estimates of the multi-stage operational amplifier circuit system.