The invention discloses a method for decomposing an out-of-core area based on a parallel high-order moment method. The method comprises the following steps of according to the actual characteristics of a solving objective, separating into n subareas, and modeling; for discontinuity, stopping to extend to the adjacent area; for continuity, respectively sectioning each subarea, and extracting model information; for each subarea, respectively solving by the high-order moment method, introducing an out-of-core solving technique when an impedance matrix is filled, and storing data by a hard disc; when the structures of the subareas are the same, filling at one time, and reusing in the subsequent iteration process; initiating the current value, setting one iteration error range and the maximum iteration times, sequentially iterating, solving the current of each subarea, and further solving the field distribution of the whole area. The method has the advantage that the accuracy, realizability and reliability are higher.