Using the data of satellite images and NCEP/NCAR 1°×1° reanalysis data , the characteristics on cloud images, track, intensity and the subsisting reaso n for a 9day longlife cold vortex are studied, and the cold air activity, humidity verti cal structure, water vapor condition and the change of thermodynamic, dynamic co nvective instability parameters related to continuous severe convective weather eve nts over the western coast of the Bohai Bay are analyzed. In addition, the key o f forecast and the threshold of severe convective weather events are approached. The mai n results show that, firstly, during 9day period the convective weather even ts appeared at south or southeast of cold vortex and their TBB values were between -40℃ to -72℃ in the severe convective process, and above -40℃ in the weak convective process. Secondly, the reason f or cold vortex subsisting is due to the transport of vorticity advection from two si des of blocking high in eastern Asia. Thirdly, the cold vortex was dynamic during longlif e cycle and the distance from the center of cold vortex to the areas studied was related to the intensity of severe convective activity. The V vector at 70050 0hPa responded sensitively to the change of shallow trough or cold air activity. Fourt hly, humidity layer at low levels becomes thicker as persistence of severe convective events, and the water vapor flux at 975hPa plays an important role in transporti ng water vapor from the Bohai Bay. Fifthly, indices SWEAT and SREH are better pa rameters than others used in the current forecast.