Abstract: First-order circuits are an important part of the basic teaching of circuit analysis. Taking the RL circuit as an example, based on the introduction of the three-factor method and analysis of the time response, the relationship between input excitation and steady-state error is analyzed through the introduction of actual component models and engineering problems. The equations of steady-state error and response time and input excitation and response time are derived respectively. Combined with the dual requirements of engineering on response time and steady-state error, the design of the input excitation source is derived, and finally the analysis of different excitation source implementation methods is expanded. The extended explanation of the first-order circuit has triggered students’ in-depth thinking and active learning, which has a certain guiding effect on teaching.