Abstract: Heterogeneous energy coupling is a distinctive feature that differentiates an integrated energy system（IES） from a single energy system such as a power distribution system. This paper proposes a measurement of the energy coupling degree in the IES,designs an observation method, and reveals new laws and mechanisms. First, this paper proposes the energy coupling indices for coupling units and entire IES. Secondly, this paper establishes the operation region model of the IES, which describes the maximum allowable operation range of the system in normal operation constraints. Finally, the proposed coupling degree is verified by a typical electricity-gas-coupled IES. This paper observes the variation law of the operation region of a electricity-gas-coupled IES in different coupling degrees, in which two critical coupling degrees and three types of phenomena are discovered. The analysis reveals the influence mechanism of coupling degree on the operation domain.