The large-scale integration of new energy units into the grid will reduce the system inertia, make the grid lack reliable inertial response, and reduce the system's frequency anti-interference ability. In this paper, by taking into account the topological coupling relationship of power networks and the dynamic characteristics of complex networks, a small disturbance stability model is constructed. And then the disturbance is modally decomposed, and the response of each node to the disturbance in the corresponding mode is analyzed to obtain the sensitivity of the system frequency stability to the node inertia variation and reveal the influence of the inertia distribution on the system frequency stability. The PSASP simulation results verify that the sensitivity of the system frequency stability is different for the change of node inertia, and the order of the sensitivity is verified. When the overall inertia of the system is limited, a reference for the position configuration of different inertia units is provided to improve the system frequency stability.