Abstract: In this paper, a study on the structure of the offshore tropical storms is made based on the data of the severe tropical storm Ida (8006) and the typhoon Ellen (8309). It is found that the remarkable features of the offshore tropical storms are the obvious asymmetry in the air-flow field and the weak warm center. A comparative diagnostic research on kinetic energy budget and heating field of such a pair of tropical storms is carried out. It is emphatically pointed out that the decay mechanism of an offshore tropical storm is implicated in the asymmetric structure. This is because the warm area and the ascending motion area are biased from the center and not coincided with each other. The baroclinic transformation process from available potential energy to kinetic energy is very weak and even negative. The another decay mechanism is that the tropical storm maintains the characteristics of a tropical system for a long time after landing and then has not rapidly changed the main energy source from latent heat release into the baroclinic process. For the tropical storms which decay soon after landing, the maximum heating is stronger and its altitude is higher.