The invention relates to a partition weak coupling degree calculating method and system for a power grid. The method comprises the following steps: converting the power grid into an edge weight graphaccording to the topology structure and active power of the power grid; calculating a laplace operator matrix of the edge weight graph and a corresponding eigenvalue and eigenvector of the laplace operator matrix; constructing partition matrixes according to the eigenvector of the laplace operator matrix; performing partition processing on the power grid based on the partition matrixes. Accordingto the method and the system, the laplace spectrum decomposition principle algorithm is improved, grid decomposition of the power grid in the normal operation state is completed based on strength of the coupling degree according to operation characteristics of the power system, and accordingly, the grid decomposition difficulty of the power grid is greatly reduced. Problems of low operation efficiency and the like of large power grid data results in setting calculation can be effectively solved with application of the method.