Topological structure of electric network is the foundation of carbon emission flow calculation and analysis. Based on that， distribution characteristics and mechanisms of carbon emission flow in electric network can be studied and the high carbon element in power systems can be revealed and identified， in order to achieve optimized decision-making. As such， two carbon emission flow distribution factors are defined to give clear representation of carbon emission flow distribution characteristics. With the proposed concept and definition of three incidence matrices， the corresponding relationship between the carbon emission from generation units and carbon emission flow in electric network can be analyzed. Thus the mechanism of the generation， transmission and consumption of carbon emission flow in electric network is revealed. The calculation methods of carbon emission incidence matrices are also given. Taking IEEE 14-bus system for case studies， the validity of this method is proven.