Based on the theory of extended equal area criterion (EEAC), the first swing of angle instability in the local power network after splitting is studied in this paper. A 3-region interconnected power system is constructed, and then, the equivalent model of the local power network after splitting is built. With the respect to the non-fault and fault-driven splitting, the change characteristics, that exist in the power-angle curve of one machine-infinity bus(OMIB) system of the local power network before and after splitting, is analyzed. Based on the computation of stability margin, the main influence factors of the first swing of angle instability are studied, according to Relative Kinetic Energy of OMIB when splitting, the acceleration area and the maximum deceleration area of the first swing. FASTEST based simulation results of the equivalent small power grid and the actual large power grid are given to show the conclusion. This work is supported by Science and Technology Project of State Grid Corporation of China "Multi-stability Morphology Interactive influence Analysis and Control Technique Research under Large Grid Serious Fault".