The invention discloses a time delay power system electromechanical oscillation mode computing method based on the SOD-PS-R (Solution Operator Discretization-Pesudo Spectrum and Rotation) algorithm. According to the relation between an eigenvalue of a time delay power system model and an operator eigenvalue of the time delay power system model, computing the eigenvalue of the time delay power system model is converted into computing the operator eigenvalue, so that a problem about computing an electromechanical oscillation mode of a time delay power system is converted into a problem about computing the operator eigenvalue; the method is used for the electromechanical oscillation mode of a large-scale time delay power system. The SOD-PS-R requires computing the maximal setting number eigenvalue of an operator discretization approximate matrix norm only, therefore one computing is enough for obtaining the electromechanical oscillation mode of the large-scale time delay power system.