Abstract: It is very important to study the economic support capability of war based on the stable operation of the national economic system so as to adapt to the high consumption and high input in the information-based war. The paper selects the defense expenditure accounts for the proportion of GDP as the index of the economic support capability of war, and establishes a simultaneous equations model which contains defense variables. The paper then adopts state equations of national economic system through state-space technique from Economic Cybernetics, and calculates the stability, controllability and observation of national economic system, and works out the economic support capability of war based on the stability of the system. The result shows that the economic support capability of war in the current period should not exceed 26.92% of GDP in the prior period. In 2013, the critical point of defense expenditure was 1 398.01 billion yuan on the premise of the stability of national economic system in China.