Abstract: To address the challenges associated with limited orbit determination accuracy, insufficient knowledge of physical parameters, and the difficulty of accurately characterizing post-impact orbital deflection for potentially Earth-impacting asteroids, near-Earth asteroid 2024 YR4 is selected as the target object to investigate orbital deflection mechanism modeling and transfer trajectory optimization for kinetic impact and close-proximity observation missions. First, a high-precision orbital propagation model is established to evaluate the impact probabilities of 2024 YR4 on the Earth and the Moon. Second, the orbital deflection behavior induced by kinetic impact is systematically investigated. The primary factors affecting the magnitude of the velocity increment are analyzed, the geometric relationship between the velocity increment direction and the Sun–asteroid–spacecraft angle is derived, and a functional relationship between the orbital deflection distance, the mean anomaly, and the velocity increment is constructed based on high-precision propagation results. On this basis, impact windows satisfying both effective deflection distance and Sun–asteroid–spacecraft angle constraints are identified, and the corresponding trajectory optimization for kinetic impact missions is carried out. Finally, for rendezvous and flyby observation missions, a year-by-year search of launch and encounter windows is performed, and feasible transfer trajectories under multiple gravity-assist schemes and flyby sequences are obtained. The modeling and analysis methods presented in this study provide a reference for the investigation of asteroid orbital deflection mechanisms, while the proposed trajectory optimization framework offers guidance for the design of future asteroid exploration and planetary defense missions.