Since the discovery of the Radiation belt, scientists have had strong interests in the magnetized space that protects us from the bombardment of solar storms. Early satellite observations show that the radiation belts consist of energetic particles and exhibit a two-belt structure. The outer belt electrons are accelerated during geomagnetic storms, while the inner-belt is mostly formed by the cosmic ray. Recently, the NASA Van Allen probes discovered a temporary storage ring that locates in the middle of the two belts when solar storm event occurs, and the radiation belts may exhibit a three-belt structure. This study aims to analyze the three-belt structure from short-term variation and long-term variation. We calculated the decay time scale in the heart of the radiation belt which describes the lifetime of the high-energy electrons. The study of the decay time is of great significance in understanding the dynamics of the whole radiation belt.