The phenomenon of anti-symmetrical bifurcation of periodic solutions occurring near an integral manifold is the intrinsic cause resulting in harmonic resonance over-voltage in power systems. Due to this discovery, the principle of eliminating resonance by using anti-bifurcation technique is presented, which makes that the theoretical bases of very measure to eliminate resonance are unified firstly from a point of view of basic theory. Our discussion models depend on a class of nonlinear control model. Using the direct Lyapunov method, a complete theoretical proof is given in accordance with the measure of eliminating resonance by connecting nonlinear resistor in series to the neutral point of P. T., and the feedback control law being applied. It comprises the action of parameters of resistor to eliminate resonance and the actual process of eliminating resonance, i.e., to go against bifurcation process which forces the big harmonic solutions to retreat to the integral manifold gradually and disappear eventually, which by using the nonlinear controllers. This makes it sure that the intrinsic cause of resonance is eliminated thoroughly.The obtained theory results and computing results are better than the presented results.