The invention discloses a power grid planning method based on a multi-core grid construction mode. The method comprises the following steps of acquiring the current power grid situation and planning boundary conditions, dividing core power grids, making the grid construction scheme of each core power grid, determining the interconnection mode among the core power grids, carrying out electrical check and analysis, making an implementation scheme of each stage and investment estimation, and outputting a target grid structure planning scheme. Through leading loads and power supplies to develop harmonically, self-balanced core power grids and the backbone grid structures are formed gradually, connection among the core power grids is simplified gradually and weakened, the target grid structure forms the structure with the core power grids as the basic units finally, each core power grid can basically realize self balancing in actual running, and the core power grids are independent relatively and can support each other. Differential planning is carried out on connection among the core power grids, the planned target grid structure has the advantages that the structure is simple and risks are dispersed, local grids can be abandoned to save the entire grid in the case of severe failure, and large-area blackout can be effectively avoided.