Two monthes after the M_S 5.2 earthquake in Chengjiang on Nov. 25, 1999, M_S5.5 earthquake occured between Qiubei and Mile, where is nearby the Chengjiang, on Jan. 27, 2000. The regional seismicity background, the seismogenic progress of the earthquake and the anomaly characteristics of seismic activity are analyzed. A case for how to predict the follow-up earthquake after a medium-strong earthquake is provided.