Identifying backbone grid is of great significance to the differential planning of power grid in that it can improve the resistance to natural disasters effectively. The stepwise graded backbone grid based on the evaluation of both equipment importance and the benefit of disaster resistance can balance disaster resistance and economy of power grid construction. This paper presents an approach to construct graded backbone according to the classification of power sources, loads and branches. Based on the life cycle cost theory, the cost calculation model of the differential construction is established with the grade of equipment, service life and depreciation value taken into account. The model of customer outage cost is set up with typical users as representatives, which, together with the benefits of power-generation-side and transmission-side and the cost calculation model, constitutes a set of economic assessment system. The feasibility of the method to search for graded backbone grid and the validity of the proposed assessment system are verified by simulation results of the IEEE 118-bus power system.