Abstract: The construction of backbone grid is the paramount condition for differential planning to improve the ability of power system on resisting natural disasters. The maximum load supplying capacity is introduced to reflect the importance of the grid lines. A new backbone grid-searching model is put forward by considering both load supply and importance of transmission line. In addition, an improved biogeography based algorithm is used to search the backbone grid. Such algorithms as mixed transport operator, chaotic mutation strategy, duplication-eliminating operations and greedy strategy are introduced to enhance the search ability, which is compared with that of the particle swarm optimization algorithm, genetic algorithm and traditional biogeography based algorithm. Simulation examples show that the proposed method is accurate and effective, the searched backbone grid scheme can meet the requirements of differential planning, and the improved biogeography based algorithm has higher search efficiency.