The forecast of national defense cost refers to the analysis and calculation of the objective expenditure process and the trend of the future defense expenditure. On the basis of judging the smoothness of defense time series, this paper uses the ADF unit root method to test the single order of time series, and uses Eviews to construct the auto-regressive AR model and moving-average MA model of national defense fee. The results show that this method is feasible and can accurately predict the level of national defense expenditures of our army in the short to medium term, which objectively and correctly reflects the law of growth of national defense expenditures in our country.