Abstract: In this paper,using a three layer model of the crustal media,the amplitude envelopes of S-waves of three component seismograms of small near earthquakes recorded by the local seismic network are studied. And we think that the regular shape of the amplitude envelope of S-waves are formed from the once-and multiple total reflections of waves at the main reflection plane. The influence of crustal layering,radiant chracteristics of the source and attenuation of crustal media on the envelope form of S-waves are analysed. On this basis,the form function of the amplitude envelope of S-waves in between the direct S-wave and the coda and the maximum amplitude of S-waves are given by theoretical calculation. Using the method of the form function of the envelope,the Q-value of crustal media in the Ningxia region is determined.