Abstract: Using NCEP (National Centers for Environmental Prediction) reanalysis data,conventional meteorological data and observational data from automatic weather stations,dynamic and thermal mechanisms of the weakening process of a cyclone over the Yangtze-Huaihe Rivers were analyzed when it moved northwards into the Bohai Sea during May 26-28,2013.The results show that intense warm advection at the northwest side of the subtropical high and cold air at the northeastern part of the Tibetan high make the cyclone moving northwards.Due to the blocking of the Japan Sea high,the cyclone moves slowly over the Bohai Sea,and the backwards-tilted structures of the high-level trough and the cyclone become forward-tilted.Negative vorticity and temperature advection behind the trough are the main reason for the declining of the cyclone.As the cyclone moves slowly over the Bohai Sea,a storm surge forms due to the long-time onshore winds and astronomical high tide.The advection of the positive temperature and vorticity in the middle and lower troposphere provide the dynamic and thermodynamic conditions for the development of a cyclone.Moist potential vorticity can reflect well the effects of absolute vorticity and atmospheric stability on the cyclone development.The diabatic heating effect occurs during the entire intensification and dissipation processes of the cyclone and mainly concentrates on the east side of the cyclone,which favors the development and maintenance of the cyclone on a certain degree.The convergence of upper-level jets and the weakening of steering flow are another reason for the cyclone moving slowly and declining over the sea.