A method to analyze the concentric relaxation vulnerability area of voltage sag based on optimal allocation of voltage sag monitors is proposed. On the basis of traditional MRA method, zero sequence impedance matrix is constructed according to the transformer connection mode and fault point location, on this basis, the amplitude of voltage sag is modified, which overcomes the shortcomings of monitoring blind spot existing in the traditional MRA method. Then the optimal allocation of the voltage sag monitoring points and fault location of the actual distribution network are realized. According to allocation of voltage sag monitors and result of fault location, the concentric relaxation vulnerability whose center is the fault point is proposed. The proposed definition can effectively reduce the workload involved in traditional voltage sag vulnerability areas and provide us a judgment reference of the attribution of liabilities between power supply department and power users. The practicability and effectiveness of the proposed method are verified by the simulation of the actual distribution network with multiple voltage levels. This work is supported by Natural Science Foundation of Shandong Province (No. ZR2012EEM026).