Abstract: To address ion beam deflection requirements for near-Earth asteroid defense, this study examines the plume characteristics and divergence mechanisms of ion thrusters, establishing a far-field plume simulation framework based on PSuM. Using the LIPS-300 thruster as a case study, far-field ion beam plume simulations are conducted for typical defense scenarios. The evolution of atomic number density distribution, ion number density and potential distribution, ion velocity distribution, and axial momentum flux in vacuum are analyzed. Results indicate that within 35 meters of the target asteroid surface, the effective thrust experienced by the asteroid can be considered equivalent to the rated thrust at the ion thruster nozzle. Beyond this distance, the effective thrust diminishes due to plume diffusion increasing with axial distance. Within 175 meters of the asteroid surface, the effective thrust generated by the ion beam can satisfy the requirement to deflect a 50-meter-class near-Earth asteroid by one Earth radius within five years. This study preliminarily verifies the engineering feasibility of ion beam deflection technology, providing reference for mission design of ion beam-based near-Earth asteroid deflection operations.