The nonlinear characteristics of Duffing circuit are researched. An experimental circuit capable of generating nonlinear phenomena is introduced. According to the Kirchhoff circuit principle, the general expression formula of Duffing circuit equation is obtained. Furthermore, MATLAB program is used to simulate the Duffing circuit equation. Simulation results show that the Duffing circuit system can present dynamical behaviors, and the simulation results are in accordance with the experimental results.