A method to reduce the model error of Hill equation in large relative distance case is proposed in this paper, by coordinate shifting to reduce the large relative distance of original system to a small one in new system. That is, as the relative distance of interceptor is up to ρ₁ (ρ₁ is defined as a controlled precision parameter, for example, ρ₁=500km), an imaginary moving reference system with its origin at the interceptor's instantanious position is set up, thus the relative motion parameters of the interceptor in original system are translated into those in the new system (obviously the initial relative distance in the new system will be ρ₍₀₎=0). As a result,the large distance problem becomes a small one，the describing precision of Hill equation is improved. This paper provides a theoretical analysis for this method and two examples of computer simulation.