Using the method of System Dynamics,the paper developed an simulational model of National S&T Infrastruc- ture.Then the structure and operation mechanism of National S&T Infrastructure was investigated through dynamic simu- lation.Policy test was also done by taking institutional construction as an example.The results indicated that System Dynamics was an effective way to explore the complex relationship in National S&T Infrastructure,and the policy tests could provide scientific evidences and powerful supports for the decision making process of National S&T Infrastructure.