This paper presents a methodology for determining the optimal transfer time in a kinetic impact defense strategy under uncertainties. Leveraging existing research, the paper first analyses the optimal deflection without considering uncertainty. It is then followed by an uncertainty-related analysis on the basis of the results. By mapping the angular parameters of the impact location, an uncertainty model is established and analytically propagated to the deflection model. The derivation of uncertainty propagation is verified with Monte Carlo simulation methods. Subsequently, the analytical expression for the deflection expectation under uncertainty is derived, and the optimal transfer time under uncertainty is investigated. The numerical results indicate that the research method proposed in this paper is effective, offering a robust approach for analyzing deflection missions under practical conditions.