In order to improve the small-signal stability of a low inertia power system, a reasonable margin of inertia should be provided to ensure the development of new energy sources, and the influence of the inertia distribution on the stability of the system with small signals should be clarified. This paper studies the influence of the inertia distribution on the inter-regional oscillation mode of the system, and constructs a Small Signal Inertia Region (SSIR) of the power system for inter-regional oscillation. First, based on the Part of the Center of Inertia (PCOI) equivalent method and the polynomial Leverrier solution method, an equivalent two-machine system interval oscillation mode damping ratio analytical formula is constructed. Then, from the critical damping ratio, the inertia domain that takes into account the small disturbance stability constraints is constructed. Further, it analyzes the relationship between the inertia distribution of the regional interconnection system and small-signal stability. Finally, a comprehensive analysis of the inertia region is carried out, and a preliminary verification is carried out in the renewable energy system. The simulation results show that the calculation method proposed in this paper can realize the accurate and rapid construction of the small signal inertia region of the power system, enhance the evaluation and monitoring ability of the small signal stability of the low inertia power system, and provide theoretical guidance for research on the renewable energy inertia region. This work is supported by the National Key Research and Development Program of China (No. 2016YFB0900100) and the National Natural Science Foundation of China (No. 52007027).