Under the background that the accuracy of voltage amplitude detection is increasing year by year, a voltage amplitude detection method based on improved second-order generalized integrator was proposed to solve the problem that the existing voltage amplitude detection methods were susceptible to the interference of harmonic and DC components, which leaded to insufficient detection accuracy. First, the composition change of voltage after fault was analyzed, and the existence of harmonic component and DC component in fault voltage was proved. Then, the principle and performance of voltage amplitude detection by sine-cosine component method and traditional second-order generalized integral method were analyzed. Then, the output of traditional second-order generalized integrator without DC component was taken as the input of the improved second-order generalized integrator’s second module. In this way, the effective filtering of the DC component was realized, and the Bode diagram, zero pole and step response of the improved second-order generalized integrator under different gain coefficients were analyzed. The optimal gain coefficient was selected considering the influence of filtering effect, stability and response speed. Finally, the simulation results show that the method proposed in this chapter has stronger filtering ability and higher precision than the other two methods.