In view of the slow response time, poor coordination and easy misoperation of the existing safety and stability measures of the power system, this paper proposes a voltage detection fault method based on the mean spectral radius (MSR).The voltage stability control cann’t be effectively implemented after the fault, and the low-voltage load shedding optimization strategy based on MSR is proposed. Firstly, according to the relationship between the MSR and the radius of the inner ring of the circle, the fault diagnosis of the AC/DC power grid is realized. If the fault is used, the voltage safety index of each load bus is calculated by the load reactive voltage response, and then the traditional low voltage load shedding is performed according to the index. The solution was improved and the real-time response information was used to enhance the control of weak voltage nodes. The method can effectively detect the fault of the power grid, and the voltage recovery value is higher after the load shedding, and the total load shedding is small, which is superior to the traditional low voltage load shedding scheme.