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