The amplitude control of limit cycle in the Qi System was implemented via a nonlinear state feedback without changing the original Hopf bifurcation point. The conditions of system parameters were obtained, which implied a Hopf bifurcation from the first classes of nontrivial equilibrium points, while a bifurcation diagram corresponding to the first class of equilibrium was drawn. By means of the center manifold theory and normal form reduction, an analytical relationship between the amplitude of limit cycle and the control gain was derived for the controlled system under a nonlinear washout filter. The comparison between the analytical solutions of amplitudes and the numerical ones were presented to confirm good predictions and valid control for amplitudes of limit cycles.