Abstract: South-yellow-sea seismic area(31°-37°N、119°-125°E) is a seismically active area for moderately strong earthquakes.The moderately strong seismic activity is due to the movement of a group of NNE and NW-NWW conjugate strike-slip fault and tensional shovel-like fault, and shows a quasi-periodicity.The epicenter takes on eastward migratory trend.Earthquake occurs at in-situor in common structure part in higher probability, its spatial distribution is heterogeneous, and its strength is greater in sea area than in land area.During the Ⅲ、Ⅳ seismically active period, moderately strong earthquakes occured in the south-pan(33°±1°、121°±1°) of south-yellow-sea mainly, which in the sea area possesses swarm and rhythm of 57-year and 6-year, and its epicenter migrates from south to north each either.The degree of seismicity of moderate earthquakes during Ⅳ seismically active period corresponds to that of small-moderate earthquakes of late years.