Previous literatures concerning about national defense budget focus on the time-series pattern and economic afford ability. These literatures aim to promote the national defense budget, but actual national defense budget decreases year by year. This study aims to construct an integrated model and uses current data to find out factors that influencing the scale of national defense budget. Factors being used in this research include one-period lag national defense budget, gross domestic products, political effect and mainland china's defense expense. The empirical data supports that one-period lag national defense budget has a significantly positive relation with national defense budget; gross domestic product and political effect have positive and negative impacts on national defense budget, respectively.