Abstract: Lg wave characteristic is widely used to discriminate underground nuclear explosions from earthquakes. In contrast, the high-frequency Lg/Pg discriminant appears to perform very well, but fails around 1 Hz. This phenomenon can not be explained by pure explosion source model, but the CLVD source appears to take a key role to explain it. In this paper, the effect of CLVD source on the performance of the discriminants based on Lg wave is studied by means of theoretical seismogram analysis. Considering the spall as an input of disturbance with special frequency, we proposed a physical mechanism of the performance of Lg/Pg based on the normal mode method. An understanding of the effect on Lg caused by CLVD source can be useful not only for improving the performance of existing regional discriminants, but also for exploring new discriminants.