The network structure and power flow of subsystem have some influence on the dynamic stability of interconnected power system. For power grid with multiple sections chain structure, a calculation method of relative sensitivity and mutual coupling degree between transmission sections in chain grid is proposed, in order to research the difference of effects of sections on power system stability and the coupled mechanism. The change law of system damping ratio after disturbance and relative angles between the delivery end and the receiving end of interconnected system, is analyzed, according to the relationship between interface power flow and angle difference at two terminals. It shows that there are several factors associating with the section sensitivity, such as equivalent reactance, the angle difference and inertia time constants of sending-end units. Based on the coupling relationship between transmission interfaces, it would be easy to coordinate interface power flow control in actual network, and it is proved effective and feasible by the results of three-machine system model and Mengxi chain network.