A diagnostic analysis is made on the weather regime at the middle stage of a severe cold wave occurring from late February to early March, 1986 by using the wavenumber domain energy equation. The energitical characteristic features are thus revealed in the process of the weather event. The analysis on the relation of the interaction between the long waves and uttra-long waves at different latitudes to the cold wave at its middle stage provides clues and criterions for the cold wave prediction in South China.