Proposed an optimal allocation method based on monitor reach matrix. Combining monitoring reach domain with the binary rules to establish monitor reach matrix, as a constraint conditions that the important nodes of all the system can be monitored. Make the least monitoring device number as the objective function, achieve the optimized configuration of the voltage sag monitoring device through the optimization algorithm with a minimum number of monitoring devices to ensure that the whole network important node’s voltage sag is observable. The voltage sag caused by all kinds of short circuit fault in the system can be captured by at least one monitoring device. Examples verify the effectiveness and practicality of the method.