Artificial radiation belts pose potential threats to spacecraft longevity and performance. High-latitude detonation points can inject large quantities of high-energy particles into Earth's outer radiation belt, which is more susceptible to geomagnetic disturbances compared to the inner radiation belt. Understanding the effects of geomagnetic activity on these particles is of significant importance. This study aims to investigate the diffusion and evolution patterns of electrons in high-L-shell artificial radiation belts under geomagnetic activity, analyzing how geomagnetic disturbances influence electron distribution and decay processes to provide theoretical foundations for spacecraft protection. A three-dimensional artificial radiation belt model was developed based on the VERB3D framework. Numerical simulations were conducted to examine electron diffusion and evolution across three parameters: radial distance, energy, and pitch angle. The analysis focused on geomagnetic effects on plasmasphere morphology, wave field intensity, and wave-particle interactions. Intense geomagnetic activity not only caused significant inward contraction of the plasmasphere but also exponentially enhanced wave field intensities both inside and outside the plasmasphere. This accelerated the diffusion process of artificial radiation belt electrons, leading to rapid flux attenuation and achieving stable distribution states in radial distance, energy, and pitch angle within a relatively short timeframe. However, under sustained geomagnetic influence, the flux of stably distributed high-energy electrons continued to decline. Geomagnetic activity can significantly accelerate the diffusion and decay processes of artificial radiation belts, thereby reducing their hazardous effects on spacecraft. These findings provide new theoretical foundations for spacecraft protection design and hold important reference value for space environment safety assurance.