Abstract: A new mechanism is brought forward to explain the tremendous relativistic electron flux enhancement events at the geo-synchronous orbit. There have been two explanations advanced before. The first theory says that the magnetic storms cause the relativistic electron flux enhancement events; according to the second one, it is the disturbance of the higher velocity branch of the solar wind that results in the events. However, in the events starting on July 14, 2000, which were studied, there had no evident disturbance of the velocity of the solar wind and the magnetic storms lagged behind the relativistic electron flux enhancement. In the present paper, we separately simulate the motions of the relativistic electrons in the x-dependent Harris-like equilibrium model and a more realistic magnetic model-T96 Model of Tsyganenko. According to the results of numerical simulation, we suggest that: when the IMF directed southward, the magnetic re-connection occurred at the dayside magnetopause. Therefore, the relativistic electrons can move to the magnetotail from the dayside magnetopause, then the electrons reached the geosynchronous orbit from the tail region by stochastic motion in the current sheet.