In recent years, the wide?band harmonic resonance has become one of the most important issues that endanger the stability of the power system. In order to build wide?band harmonic resonance oscillation monitoring system, an optimal placement method of monitor device is proposed based on the power grid topology. Firstly, this method defines the criticality of comprehensive nodes from the perspective of the harmonic resonance observability. Then, under the premise of ensuring the overall observability of the power system in the event of N?1 contingencies on critical transmission lines, a 0?1 integer programming mathematical model is constructed for the goal of monitoring device configurations number minimization and the criticality maximization. The optimal placement scheme is obtained by using the improved binary particle swarm algorithm to solve the model. Finally, the reliability and economy of the proposed algorithm are verified through the simulation tests of the IEEE 14?bus system and the IEEE 30?bus system.