The potential hazard of asteroids on the impact of Earth has drawn increasing attention in recent years. The instantaneous moment alteration via impact is the most feasible deflection technique. However, there are a number of factors that influence the impact of progress. Some of them are still not understood, which causes a large uncertainty of the change of velocity after impact and affects the performance of orbit deflection. In this work, we investigate the effects of planetary perturbation and orbital uncertainty on the defense effectiveness of kinetic energy impact missions. Based on the real ephemeris model, a dynamic model considering the gravitational perturbation of many bodies is established. A virtual asteroid orbit with Earth impact characteristics is constructed by the inverse orbit integration method. the state transition tensor (STT) method is employed to calculate the influence of the impact direction uncertainty on orbit deflection. Research results demonstrate that the optimal asteroid impact moment appears in perihelion, which not only enhances orbital dis-placement but also suppresses perturbations in deflection outcomes induced by velocity direction errors. Crucially, our study also reveals that close planets flyby after impact will also influence the deflection performance significantly. This suggests that specialized orbital analysis of such gravitational interactions is necessary in actual mission design to ensure a reliable defense strategy.