The countries under the disturbed-reactive environment have apparent enemy and disturbed-reactive behavior. While put enormous resources in defense sector to find the military equilibrium, these countries also develop their non-defensive projects, Thus, the extremely difficulty determining defense budget size is resulted. This paper indicates the planning strategy for defense budget size for the countries under this environment. Through the application of system dynamic model, we can understand the impact of distinct defense budget planning strategy on the defense budget size. In addition, the defense budget planning strategy of ROC is figured out.