Abstract: A function for multi-department production is established, and used to analyze the causes of demand under the level of full employment. It comes to a conclusion that demand under the level of full employment is partly due to the supply of economics and partly due to the demand. According to the causes, total supply countermeasures and total demand countermeasures are put forward,i. e., implementing effective financial policies, perfecting social security mechanism, adjusting revenue polices, and increasing exports,etc.