There are controversies on ground motion attenuation relations at the epicentral distance of about 100kin. Some models predict that the seismic energy remains almost constant or becomes even stronger with increasing distance at this distance, while other models hold that it decreases with increased distance. The divergence lies mainly in whether SmS is stronger than direct S and surface waves at this distance. With the Ms5. 9 earthquake sequence in the Yingjiang region of Yunnan Province as an example, we demonstrate that SmS is always 2～5 times stronger than direct S waves around the epicentral distance of 100kin (which is the post-critical distance for S in this region). Study of synthetic seismograms suggest that crustal structure has an important effect on the amplitude of post-critical SmS, with simple crust producing strong SmS. This preliminary study confirms that in China, SmS also plays an important role on ground motion at distances around 100km, which demands more studies of post-critical SmS.