The linear interpolation method commonly used in intelligent substations is researched, and an improved method is proposed for the problem of amplitude attenuation. First of all, the amplitude gain formula of linear interpolation is derived by using the discrete transfer function. Then the amplitude frequency characteristic of linear interpolation is analyzed, and the influence of amplitude attenuation on the protection and measurement is evaluated. In the 4 kHz sampling rate, the linear interpolation can not meet the requirement of harmonic accuracy. On this basis, an improved interpolation method is proposed, using the symmetry of the amplitude gain about the interpolation position. The theoretical proof shows that the amplitude gain of each frequency signal is 1. Additionally, the software implementation process is given, particularly the selection of the initial value, and the stability of the method is analyzed. Finally, the interpolation effect of the method is verified by FFT analysis. The results show that the improved method can solve the problem of amplitude attenuation of linear interpolation.