Abstract: According to the dynamical behavior of seismic activity and the spatio-temporal complexity of seismic system, this paper analyzed the seismic activities from the perspective of complex network. Using the solution based on space-time influence domain to construct earthquake network, a method of k-core decomposition was applied to the earthquake networks. The experiment result shows that the nodes in the highest layer cluster together. An important phenomenon is found that the strong quakes tend to occur at the location where the nodes in the highest layer are by analyzing their location information. What is more, the value of the highest layer will rapidly increase before a strong quake, then it will reach the maximum when a strong quake occurs, finally it will return to a stable state. k-core decomposition of earthquake networks will provide a new way for us to analyze and research the seismological system.