Abstract: This article focuses on the defense against kinetic impact of the near-Earth asteroid 2024 YR4 and investigates the effect of orbital deflection under velocity increment deviation. Firstly, an orbital evolution model integrating planetary perturbations and impact errors is established. Secondly, the influence of velocity increment error on the orbital deflection effect of the asteroid is systematically analyzed by the semi - analytical method of State Transition Tensor (STT). Finally, the differences in the evolution of the major axis of the error ellipsoid after impact at different times are analyzed. The study finds that the evolution of orbital uncertainty caused by the initial velocity increment error is dominated by the orbital phase, and the magnitude of the velocity increment only affects the error amplitude rather than the propagation form. When implementing impacts at different initial moments and evolving for the same duration, the difference in the size of the major axis of the error ellipsoid can reach 3.7 times, indicating that the timing of the impact has a significant impact on the uncertainty of the orbital deflection effect. In addition, the analysis found that the Earth Star rendezvous event in 2028 will alter the expected deflection. This study provides a reference for developing asteroid defense strategies that consider error propagation.