Abstract: Spall is defined as a secondary seismic source contributing to regional and far field seismic signals generated by underground nuclear explosions.It can modulate the regional phase,i.e.,Lg waves.This paper analyzed the characteristics of Lg waves excited by explosion source and spall based on three typical crustal models by using numerical simulation of theoretical seismograms,then obtained the relationship between the crustal velocity models and the Lg waves excited by explosion source and spall.To explain the generation mechanism of Lg waves,we investigated the variation of S waves induced directly by compensate linear vector dipole (CLVD) source with different epicentral distances.The results indicate that CLVD source plays an important role in generating low-frequency Lg waves by underground nuclear explosions.