This research makes use of modeling procedure of System Dynamics to construct the offshore purchasing non-active duty weapon system's spare parts inventory policy dynamic model, which attempts the explanation that the offshore purchasing non-active duty weapon system's spare parts obtains the system behavior between the possibility developing and weapon system availability. Through adjusting the critical variables to simulate the feasibility of the spare parts inventory policy, In view of ”sells separately accepts makes up” feasibility of simulation discovery the inventory policy, by stop using the mechanism which the equipment sells separately accepts makes up, may cause spare parts to make up the reserve and available equipment quantity increases properly along with it, the spare parts purchase's cost reduces along with it.