Electromagnetic launch technology accelerates projectiles through electromagnetic forces. The projectiles’ performance significantly influenced by the flight stability of the projectiles. This paper focuses on a rotational armature electromagnetic launch system, employing an asymmetric additional rail structure to analyze its electromagnetic field characteristics and armature rotation mechanisms via numerical simulation. A three-dimensional transient model was established using LS-DYNA to investigate the kinematic Characteristics of the armature, current density distribution, magnetic field evolution, and Lorentz force distribution under the influence of additional rails. The simulation results demonstrate that the additional rails significantly enhance the inductance gradient, enabling the armature to achieve a muzzle velocity of 600 m/s and a clockwise rotational speed of 205,274 r/min. The current density, affected by skin and proximity effects, concentrates on the inner edges of the rails and the outer edges of the armature, exhibiting an asymmetric bias. The magnetic field intensity increases with the armature’s velocity, forming a “trailing” diffusion structure at high speed. The Lorentz force predominantly concentrates at the contact edges between the armature and rails, with its asymmetric distribution identified as the key driver of armature rotation. This study reveals the regulatory mechanisms of additional rails on electromagnetic field distribution and armature motion characteristics, providing a theoretical foundation for optimizing the design of rotational armature electromagnetic launch systems.