Abstract: On the basis of the diagnostic study for 1998～2000 cold episode,this paper revealed the main features such as rapid onset,great intensity,quick return to near normal condition of SST,and slow response of the atmospheric condition. Compared with the other cold episodes,the ocean subsurface temperatures and winds were analyzed as the main triggering factors for the onset cold episode. Results indicted that the enhancement and eastward development of the subsurface temperatures are the bases of the onset of cold episode. Change of sea surface winds played crucial roles in the onset and development of cold episode. The onset season impacts on the duration of the episode.