Abstract: For interdisciplinary journals, there often exist great differences (from severalfold to several tenfold, or even to more than a hundredfold) among journal impact factors.In order to find why there is so large difference in impact factors among different disciplines and how to solve this problem, and to make journal impact factors reflect the difference of influence among not only intradisciplinary journals but also interdisciplinary journals, this paper theoretically studied the influence of journals' number on journals' impact factor based on the JCR data.The research results show that as a whole, the values of journal impact factors increase with the quantity of intradisciplinary journals increasing, and there is no much sense for comparing the impact factor of interdisciplinary journals.A new improved method for calculating journal impact factors was proposed in this paper, which can obviously discount the dependence of impact factors on journals' number, and the improved journal impact factors can effectively reflect the significance of journals and make the interdisciplinary comparison of journals become more valuable.