High-altitude nuclear explosions can inject large amounts of relativistic electrons into the inner magnetosphere, resulting in the formation of artificial radiation belts. These high-energy electrons pose a potential threat to spacecraft due to their long-term stability and influence on space weather. The investigation of the formation and evolution of artificial radiation belts is of great significance for the safety of spacecraft and human space activities. In this study, the comprehensive inner magnetosphere-ionosphere (CIMI) model is used to simulate the transition of electrons from a locally concentrated distribution to an azimuthally uniform distribution, which reveals the spiral encircling, azimuthal expansion, and diffusion behaviors exhibited by the electron cloud during the formation of artificial radiation belts. The CIMI model is a four-dimensional model based on the Fokker-Planck equation. It simulates the evolution of particles across four degrees of freedom: radial, azimuthal, energy, and equatorial pitch angle. Unlike previous studies that mainly focus on the long-term evolution of artificial radiation belts already reaching azimuthal uniformity, this work specifically ascertains the azimuthal evolution process of the injected electrons and how they form the artificial radiation belts. Numerical simulations are conducted on the captured nuclear explosion electrons initially concentrated at L = 1.1–2.2 and covering approximately one time zone azimuthally. The results show that the injected electrons primarily evolve into an azimuthally uniform distribution through a spiral encircling process, with diffusion playing a smaller role. In this process, the electrons undergo eastward drift, with those at higher altitudes exhibiting faster drift velocities. The velocity shear leads to the formation of a helical structure around the Earth. Additionally, the formation of this spiral structure is accompanied by azimuthal expansion, driven mainly by energy and pitch angle dispersion during the drift. Electrons with different energy values and equatorial pitch angles exhibit varying drift speeds, contributing to the azimuthal expansion of electron clusters during the drift. The expansion process can fill the gaps in the helical structure. Ultimately, the electron distribution achieves azimuthal uniformity through energy-pitch angle diffusion.