A method of optimal partitioning selection scheme based on “partition+ searching” is proposed for the timely controlled islanding of instable large-scale power system. Firstly, the variation quantity of voltage amplitude and phase angle are used as feature vector, and the power grid structure is divided into stability region, disability region and islanding region by fuzzy c-means algorithm. Then, the Edmonds-Karp algorithm and Dijkstra algorithm are used to calculate the optimal partitioning section with the objective of minimum power flow impact. This method needs no reduction of the whole power network with lower computation complexity. Therefore, the optimal section can be obtained online. Simulation results compared with traditional methods on the IEEE 118 panel points system and a reality power grid prove that this partitioning scheme is correct, effective and fast.