This paper first puts forward the concept of “Interactive Strength”, which denotes the closeness degree of communication, cooperation, competition and learning. Furthermore, this paper to use the interactive strength to define the bounds of the regional innovation system. As an innovational system, the most important factor of a region is the interactive relationship among individuals and the network, thus the measurement of interactive strength quantify such relationship, which lays a solid foundation for the definition of the bounds of regional innovation system.