Voltage sags are unavoidable and seriously harmful to sensitive loads. Reasonable configuration of limited monitors can reduce monitoring costs,provide data support for voltage sag management and reduction of voltage sag hazards. Therefore,the optimal configuration of voltage sag monitors is important. Aiming at the traditional method that ignores the inconsistency of the degree of voltage sag damage in different areas,a joint optimal configuration model of voltage sag monitor considering the monitoring reliability of sag sensitive area and the location of phasor measurement unit (PMU) is proposed. Considering the considerable sag as the constraint condition,the minimum number of monitors and the widest coverage of the sag sensitive area are used as the goals to form an optimized configuration model. In this model,the PMU is configured to assist in monitoring voltage sags. In addition,the Zeroin method is used to improve the accuracy of the exposed area calculation. The IEEE 30-bus system is used in simulation to prove that the difficulty of balancing the monitoring cost with the monitoring capability of sensitive areas is overcome by the proposed method. The proposed method can ensure the minimum number of monitors and high redundancy coverage of sensitive loads,thus the economic benefits.