Abstract: The case selected in this study is an intense typhoon (Billie) which moved over the Southeast China and interacted with an extratropical frontal system. Kinetic energy(KE) budget and available potential energy (APE) were computed by using the quasi-Lagrangian scheme during the period 10 to 12 Aug. 1976. The main results of this study are: 1)In the weakening process of the landed typhoon, exchange of KE with the environment was small. Then the typhoon could be considered as a "quasi-closed" system. 2) In the various weakening stages of the landed typhoon, the thermodynamic and dynamic processes were different. When the typhoon moved just over land and accompanied with heavy rain, the latent heat and moist convection were still the major source of KE. The energy sink due to cross-contour flow in the upper troposphere was the important reason for the decrepitude of the typhoon. The barotropic process was dominant in the decrease of KE. However, after the typhoon interacted with the westerlies, the KE generation by baroclinic process increased and the frictional dissipation played a significant role in this stage. 3) Throughout the period of interest, large amount of APE entered into the environmental atmosphere through the lateral boundaries of the typhoon circulation. This may be one of the major processes of interaction between the typhoon and the surroundings.