By the literature study and essential judgment of think tanks’ influence, and applying the ideas from mass communication, innovation diffusion, and political analysis, from the triality of “subject-target-environment”, this paper proposes six fundamental factors of think tanks’ influence, i.e. social capital, thought product, audient target, influence channel, intervention opportunity and product market. It constructs a model for these factors, and studies the cases of the Academy of Sciences - U.S.A., the Royal Society - G. B., and Academy of Sciences - India. Finally, it concludes that think tanks’ influence is the result of synthetical operations of resource factor, terminal factor, target factor, path factor, time factor and space factor.