style="MARGIN: 0cm 0cm 0pt" class=MsoNormal>style="FONT-SIZE: 14pt; mso-font-kerning: 0pt" lang=EN-US>The authors&nbsp;study the variation regularity of wave-velocity ratio in the 4th active period of strong earthquakes in space prefix = ns0 ns = "urn:schemas-microsoft-com:office:smarttags" />s0:place>s0:PlaceName>space prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /><st1:place w:st="on"><st1:PlaceName w:st="on">Yunnanst1:PlaceName>s0:PlaceName>st1:place> s0:PlaceType><st1:PlaceType w:st="on">Provincest1:PlaceType>s0:PlaceType>s0:place>, and discuss the relation between strong earthquake genesis and variation of wave velocity, medium-small earthquakes, and variation of focal depth. The result shows that corresponding to the beginning and the ending of the 4th active period, wave-velocity ratio in s0:State>s0:place>Yunnans0:place>s0:State> Province experienced the quasi-synchronous declining and rising.space prefix = o ns = "urn:schemas-microsoft-com:office:office" />