Abstract: To online prevent or suppress the low-frequency oscillation, the sensitivity of eigenvalue to operating parameter is calculated. It is applied to generate the effective control strategy that can improve the system damp. At the beginning, Fuzzy ISODATA clustering method was utilized to obtain the coherent grouping relation of the buses, and then the controlled generators were screened by participation factor. With the efficient parallel computation of eigenvalue sensitivity, the gradients of the quantitative index of power system small signal stability（damping ratio） to control variables（generator output） could be obtained and used to direct generation dispatch. The control strategy satisfied many constraints, such as power limitation and the operability for dispatch operation. To solve 20 000 nodes scale power system problem, the time cost was less than 5 minutes. The effectiveness and feasibility of the proposed method were demonstrated using an actual power system of China. The online evaluation module using the proposed method was embedded in an online early warning and security assessment system for real practice.